/usr/man/cat.3/DBI.3.Z(/usr/man/cat.3/DBI.3.Z)
NAME
DBI - Database independent interface for Perl
SYNOPSIS
use DBI;
@driver_names = DBI->available_drivers;
@data_sources = DBI->data_sources($driver_name, \%attr);
$dbh = DBI->connect($data_source, $username, $auth, \%attr);
$rv = $dbh->do($statement);
$rv = $dbh->do($statement, \%attr);
$rv = $dbh->do($statement, \%attr, @bind_values);
$ary_ref = $dbh->selectall_arrayref($statement);
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement);
$ary_ref = $dbh->selectrow_arrayref($statement);
$hash_ref = $dbh->selectrow_hashref($statement);
$sth = $dbh->prepare($statement);
$sth = $dbh->prepare_cached($statement);
$rc = $sth->bind_param($p_num, $bind_value);
$rc = $sth->bind_param($p_num, $bind_value, $bind_type);
$rc = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute;
$rv = $sth->execute(@bind_values);
$rc = $sth->bind_param_array($p_num, $bind_values, \%attr);
$rv = $sth->execute_array(\%attr);
$rv = $sth->execute_array(\%attr, @bind_values);
$rc = $sth->bind_col($col_num, \$col_variable);
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
@row_ary = $sth->fetchrow_array;
$ary_ref = $sth->fetchrow_arrayref;
$hash_ref = $sth->fetchrow_hashref;
$ary_ref = $sth->fetchall_arrayref;
$ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
$hash_ref = $sth->fetchall_hashref( $key_field );
$rv = $sth->rows;
$rc = $dbh->begin_work;
$rc = $dbh->commit;
$rc = $dbh->rollback;
$quoted_string = $dbh->quote($string);
$rc = $h->err;
$str = $h->errstr;
$rv = $h->state;
$rc = $dbh->disconnect;
This synopsis above only lists the major methods.
GETTING HELP
If you have questions about DBI, you can get help from the
dbi-users@perl.org mailing list. You can get help on subscribing and
using the list by emailing:
dbi-users-help@perl.org
Also worth a visit is the DBI home page at:
http://dbi.perl.org/
Before asking any questions, reread this document, consult the archives
and read the DBI FAQ. The archives are listed at the end of this docu-
ment and on the DBI home page. The FAQ is installed as a DBI::FAQ mod-
ule so you can read it by executing "perldoc DBI::FAQ".
To help you make the best use of the dbi-users mailing list, and any
other lists or forums you may use, I strongly recommend that you read
"How To Ask Questions The Smart Way" by Eric Raymond:
http://www.tuxedo.org/~esr/faqs/smart-questions.html
This document often uses terms like references, objects, methods. If
you're not familar with those terms then it would be a good idea to
read at least the following perl manuals first: perlreftut, perldsc,
perllol, and perlboot.
Please note that Tim Bunce does not maintain the mailing lists or the
web page (generous volunteers do that). So please don't send mail
directly to him; he just doesn't have the time to answer questions per-
sonally. The dbi-users mailing list has lots of experienced people who
should be able to help you if you need it. If you do email Tim he's
very likely to just forward it to the mailing list.
NOTES
This is the DBI specification that corresponds to the DBI version 1.32
("$Date: 2002/12/01 22:34:29 $").
The DBI is evolving at a steady pace, so it's good to check that you
have the latest copy.
The significant user-visible changes in each release are documented in
the DBI::Changes module so you can read them by executing "perldoc
DBI::Changes".
Some DBI changes require changes in the drivers, but the drivers can
take some time to catch up. Recent versions of the DBI have added new
features (generally marked NEW in the text) that may not yet be sup-
ported by the drivers you use. Talk to the authors of those drivers if
you need the new features.
Extensions to the DBI API often use the "DBIx::*" namespace. See "Nam-
ing Conventions and Name Space" and:
http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
DESCRIPTION
The DBI is a database access module for the Perl programming language.
It defines a set of methods, variables, and conventions that provide a
consistent database interface, independent of the actual database being
used.
It is important to remember that the DBI is just an interface. The DBI
is a layer of "glue" between an application and one or more database
driver modules. It is the driver modules which do most of the real
work. The DBI provides a standard interface and framework for the
drivers to operate within.
Architecture of a DBI Application
|<- Scope of DBI ->|
.-. .--------------. .-------------.
.-------. | |---| XYZ Driver |---| XYZ Engine |
| Perl | | | `--------------' `-------------'
| script| |A| |D| .--------------. .-------------.
| using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
| DBI | |I| |I| `--------------' `-------------'
| API | | |...
|methods| | |... Other drivers
`-------' | |...
`-'
The API, or Application Programming Interface, defines the call inter-
face and variables for Perl scripts to use. The API is implemented by
the Perl DBI extension.
The DBI "dispatches" the method calls to the appropriate driver for
actual execution. The DBI is also responsible for the dynamic loading
of drivers, error checking and handling, providing default implementa-
tions for methods, and many other non-database specific duties.
Each driver contains implementations of the DBI methods using the pri-
vate interface functions of the corresponding database engine. Only
authors of sophisticated/multi-database applications or generic library
functions need be concerned with drivers.
Notation and Conventions
The following conventions are used in this document:
$dbh Database handle object
$sth Statement handle object
$drh Driver handle object (rarely seen or used in applications)
$h Any of the handle types above ($dbh, $sth, or $drh)
$rc General Return Code (boolean: true=ok, false=error)
$rv General Return Value (typically an integer)
@ary List of values returned from the database, typically a row of data
$rows Number of rows processed (if available, else -1)
$fh A filehandle
undef NULL values are represented by undefined values in Perl
\%attr Reference to a hash of attribute values passed to methods
Note that Perl will automatically destroy database and statement handle
objects if all references to them are deleted.
Outline Usage
To use DBI, first you need to load the DBI module:
use DBI;
use strict;
(The "use strict;" isn't required but is strongly recommended.)
Then you need to "connect" to your data source and get a handle for
that connection:
$dbh = DBI->connect($dsn, $user, $password,
{ RaiseError => 1, AutoCommit => 0 });
Since connecting can be expensive, you generally just connect at the
start of your program and disconnect at the end.
Explicitly defining the required "AutoCommit" behavior is strongly rec-
ommended and may become mandatory in a later version. This determines
whether changes are automatically committed to the database when exe-
cuted, or need to be explicitly committed later.
The DBI allows an application to "prepare" statements for later execu-
tion. A prepared statement is identified by a statement handle held in
a Perl variable. We'll call the Perl variable $sth in our examples.
The typical method call sequence for a "SELECT" statement is:
prepare,
execute, fetch, fetch, ...
execute, fetch, fetch, ...
execute, fetch, fetch, ...
for example:
$sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");
$sth->execute( $baz );
while ( @row = $sth->fetchrow_array ) {
print "@row\n";
}
The typical method call sequence for a non-"SELECT" statement is:
prepare,
execute,
execute,
execute.
for example:
$sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");
while(<CSV>) {
chomp;
my ($foo,$bar,$baz) = split /,/;
$sth->execute( $foo, $bar, $baz );
}
The "do()" method can be used for non repeated non-"SELECT" statement
(or with drivers that don't support placeholders):
$rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");
To commit your changes to the database (when "AutoCommit" is off):
$dbh->commit; # or call $dbh->rollback; to undo changes
Finally, when you have finished working with the data source, you
should "disconnect" from it:
$dbh->disconnect;
General Interface Rules & Caveats
The DBI does not have a concept of a "current session". Every session
has a handle object (i.e., a $dbh) returned from the "connect" method.
That handle object is used to invoke database related methods.
Most data is returned to the Perl script as strings. (Null values are
returned as "undef".) This allows arbitrary precision numeric data to
be handled without loss of accuracy. Beware that Perl may not preserve
the same accuracy when the string is used as a number.
Dates and times are returned as character strings in the current
default format of the corresponding database engine. Time zone effects
are database/driver dependent.
Perl supports binary data in Perl strings, and the DBI will pass binary
data to and from the driver without change. It is up to the driver
implementors to decide how they wish to handle such binary data.
Most databases that understand multiple character sets have a default
global charset. Text stored in the database is, or should be, stored in
that charset; if not, then that's the fault of either the database or
the application that inserted the data. When text is fetched it should
be automatically converted to the charset of the client, presumably
based on the locale. If a driver needs to set a flag to get that behav-
ior, then it should do so; it should not require the application to do
that.
Multiple SQL statements may not be combined in a single statement han-
dle ($sth), although some databases and drivers do support this
(notably Sybase and SQL Server).
Non-sequential record reads are not supported in this version of the
DBI. In other words, records can only be fetched in the order that the
database returned them, and once fetched they are forgotten.
Positioned updates and deletes are not directly supported by the DBI.
See the description of the "CursorName" attribute for an alternative.
Individual driver implementors are free to provide any private func-
tions and/or handle attributes that they feel are useful. Private
driver functions can be invoked using the DBI "func()" method. Private
driver attributes are accessed just like standard attributes.
Many methods have an optional "\%attr" parameter which can be used to
pass information to the driver implementing the method. Except where
specifically documented, the "\%attr" parameter can only be used to
pass driver specific hints. In general, you can ignore "\%attr" parame-
ters or pass it as "undef".
Naming Conventions and Name Space
The DBI package and all packages below it ("DBI::*") are reserved for
use by the DBI. Extensions and related modules use the "DBIx::" names-
pace (see "http://www.perl.com/CPAN/modules/by-module/DBIx/"). Package
names beginning with "DBD::" are reserved for use by DBI database
drivers. All environment variables used by the DBI or by individual
DBDs begin with ""DBI_"" or ""DBD_"".
The letter case used for attribute names is significant and plays an
important part in the portability of DBI scripts. The case of the
attribute name is used to signify who defined the meaning of that name
and its values.
Case of name Has a meaning defined by
------------ ------------------------
UPPER_CASE Standards, e.g., X/Open, ISO SQL92 etc (portable)
MixedCase DBI API (portable), underscores are not used.
lower_case Driver or database engine specific (non-portable)
It is of the utmost importance that Driver developers only use lower-
case attribute names when defining private attributes. Private
attribute names must be prefixed with the driver name or suitable
abbreviation (e.g., ""ora_"" for Oracle, ""ing_"" for Ingres, etc).
Driver Specific Prefix Registry:
ad_ DBD::AnyData
ado_ DBD::ADO
best_ DBD::BestWins
csv_ DBD::CSV
db2_ DBD::DB2
df_ DBD::DF
f_ DBD::File
file_ DBD::TextFile
ib_ DBD::InterBase
ing_ DBD::Ingres
ix_ DBD::Informix
msql_ DBD::mSQL
mysql_ DBD::mysql
odbc_ DBD::ODBC
ora_ DBD::Oracle
pg_ DBD::Pg
proxy_ DBD::Proxy
rdb_ DBD::RDB
sapdb_ DBD::SAP_DB
solid_ DBD::Solid
sql_ SQL::Statement (used by some drivers)
syb_ DBD::Sybase
tdat_ DBD::Teradata
tmpl_ DBD::Template
tmplss_ DBD::TemplateSS
tuber_ DBD::Tuber
uni_ DBD::Unify
xbase_ DBD::XBase
xl_ DBD::Excel
SQL - A Query Language
Most DBI drivers require applications to use a dialect of SQL (Struc-
tured Query Language) to interact with the database engine. The fol-
lowing links provide useful information and further links about SQL:
http://www.altavista.com/query?q=sql+tutorial
http://www.jcc.com/sql_stnd.html
http://www.contrib.andrew.cmu.edu/~shadow/sql.html
The DBI itself does not mandate or require any particular language to
be used; it is language independent. In ODBC terms, the DBI is in
"pass-thru" mode, although individual drivers might not be. The only
requirement is that queries and other statements must be expressed as a
single string of characters passed as the first argument to the "pre-
pare" or "do" methods.
For an interesting diversion on the real history of RDBMS and SQL, from
the people who made it happen, see:
http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html
Follow the "And the rest" and "Intergalactic dataspeak" links for the
SQL history.
Placeholders and Bind Values
Some drivers support placeholders and bind values. Placeholders, also
called parameter markers, are used to indicate values in a database
statement that will be supplied later, before the prepared statement is
executed. For example, an application might use the following to
insert a row of data into the SALES table:
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
or the following, to select the description for a product:
SELECT description FROM products WHERE product_code = ?
The "?" characters are the placeholders. The association of actual
values with placeholders is known as binding, and the values are
referred to as bind values.
When using placeholders with the SQL "LIKE" qualifier, you must remem-
ber that the placeholder substitutes for the whole string. So you
should use ""... LIKE ? ..."" and include any wildcard characters in
the value that you bind to the placeholder.
Null Values
Undefined values, or "undef", can be used to indicate null values.
However, care must be taken in the particular case of trying to use
null values to qualify a "SELECT" statement. Consider:
SELECT description FROM products WHERE product_code = ?
Binding an "undef" (NULL) to the placeholder will not select rows which
have a NULL "product_code"! Refer to the SQL manual for your database
engine or any SQL book for the reasons for this. To explicitly select
NULLs you have to say ""WHERE product_code IS NULL"" and to make that
general you have to say:
... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL))
and bind the same value to both placeholders. Sadly, that more general
syntax doesn't work for Sybase and MS SQL Server. However on those two
servers the original ""product_code = ?"" syntax works for binding
nulls.
Performance
Without using placeholders, the insert statement shown previously would
have to contain the literal values to be inserted and would have to be
re-prepared and re-executed for each row. With placeholders, the insert
statement only needs to be prepared once. The bind values for each row
can be given to the "execute" method each time it's called. By avoiding
the need to re-prepare the statement for each row, the application typ-
ically runs many times faster. Here's an example:
my $sth = $dbh->prepare(q{
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
}) or die $dbh->errstr;
while (<>) {
chomp;
my ($product_code, $qty, $price) = split /,/;
$sth->execute($product_code, $qty, $price) or die $dbh->errstr;
}
$dbh->commit or die $dbh->errstr;
See "execute" and "bind_param" for more details.
The "q{...}" style quoting used in this example avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote like
"qq{...}" operator if you want to interpolate variables into the
string. See "Quote and Quote-like Operators" in perlop for more
details.
See also the "bind_column" method, which is used to associate Perl
variables with the output columns of a "SELECT" statement.
THE DBI PACKAGE AND CLASS
In this section, we cover the DBI class methods, utility functions, and
the dynamic attributes associated with generic DBI handles.
DBI Constants
Constants representing the values of the SQL standard types can be
imported individually by name, or all together by importing the special
":sql_types" tag.
The names and values of all the defined SQL standard types can be pro-
duced like this:
foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
printf "%s=%d\n", $_, &{"DBI::$_"};
}
These constants are defined by SQL/CLI, ODBC or both. "SQL_BIGINT" is
(currently) omitted, because SQL/CLI and ODBC provide conflicting
codes.
See the "type_info", "type_info_all", and "bind_param" methods for pos-
sible uses.
Note that just because the DBI defines a named constant for a given
data type doesn't mean that drivers will support that data type.
DBI Class Methods
The following methods are provided by the DBI class:
"connect"
$dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect($data_source, $username, $password, \%attr)
or die $DBI::errstr;
Establishes a database connection, or session, to the requested
$data_source. Returns a database handle object if the connection
succeeds. Use "$dbh-">"disconnect" to terminate the connection.
If the connect fails (see below), it returns "undef" and sets both
$DBI::err and $DBI::errstr. (It does not set $!, etc.) You should
generally test the return status of "connect" and "print
$DBI::errstr" if it has failed.
Multiple simultaneous connections to multiple databases through
multiple drivers can be made via the DBI. Simply make one "connect"
call for each database and keep a copy of each returned database
handle.
The $data_source value should begin with ""dbi:"driver_name":"".
The driver_name specifies the driver that will be used to make the
connection. (Letter case is significant.)
As a convenience, if the $data_source parameter is undefined or
empty, the DBI will substitute the value of the environment vari-
able "DBI_DSN". If just the driver_name part is empty (i.e., the
$data_source prefix is ""dbi::""), the environment variable
"DBI_DRIVER" is used. If neither variable is set, then "connect"
dies.
Examples of $data_source values are:
dbi:DriverName:database_name
dbi:DriverName:database_name@hostname:port
dbi:DriverName:database=database_name;host=hostname;port=port
There is no standard for the text following the driver name. Each
driver is free to use whatever syntax it wants. The only require-
ment the DBI makes is that all the information is supplied in a
single string. You must consult the documentation for the drivers
you are using for a description of the syntax they require. (Where
a driver author needs to define a syntax for the $data_source, it
is recommended that they follow the ODBC style, shown in the last
example above.)
If the environment variable "DBI_AUTOPROXY" is defined (and the
driver in $data_source is not ""Proxy"") then the connect request
will automatically be changed to:
$ENV{DBI_AUTOPROXY};dsn=$data_source
"DBI_AUTOPROXY" is typically set as ""dbi:Proxy:host-
name=...;port=..."". If $ENV{DBI_AUTOPROXY} doesn't begin with
'"dbi:"' then "dbi:Proxy:" will be prepended to it first. See the
DBD::Proxy documentation for more details.
If $username or $password are undefined (rather than just empty),
then the DBI will substitute the values of the "DBI_USER" and
"DBI_PASS" environment variables, respectively. The DBI will warn
if the environment variables are not defined. However, the every-
day use of these environment variables is not recommended for secu-
rity reasons. The mechanism is primarily intended to simplify test-
ing.
"DBI-">"connect" automatically installs the driver if it has not
been installed yet. Driver installation either returns a valid
driver handle, or it dies with an error message that includes the
string ""install_driver"" and the underlying problem. So
"DBI-">"connect" will die on a driver installation failure and will
only return "undef" on a connect failure, in which case
$DBI::errstr will hold the error message.
The $data_source argument (with the ""dbi:...:"" prefix removed)
and the $username and $password arguments are then passed to the
driver for processing. The DBI does not define any interpretation
for the contents of these fields. The driver is free to interpret
the $data_source, $username, and $password fields in any way, and
supply whatever defaults are appropriate for the engine being
accessed. (Oracle, for example, uses the ORACLE_SID and TWO_TASK
environment variables if no $data_source is specified.)
The "AutoCommit" and "PrintError" attributes for each connection
default to "on". (See "AutoCommit" and "PrintError" for more infor-
mation.) However, it is strongly recommended that you explicitly
define "AutoCommit" rather than rely on the default. Future ver-
sions of the DBI may issue a warning if "AutoCommit" is not explic-
itly defined.
The "\%attr" parameter can be used to alter the default settings of
"PrintError", "RaiseError", "AutoCommit", and other attributes. For
example:
$dbh = DBI->connect($data_source, $user, $pass, {
PrintError => 0,
AutoCommit => 0
});
You can also define connection attribute values within the
$data_source parameter. For example:
dbi:DriverName(PrintError=>0,Taint=>1):...
Individual attributes values specified in this way take precedence
over any conflicting values specified via the "\%attr" parameter to
"connect".
The "dbi_connect_method" attribute can be used to specify which
driver method should be called to establish the connection. The
only useful values are 'connect', 'connect_cached', or some spe-
cialized case like 'Apache::DBI::connect' (which is automatically
the default when running within Apache).
Where possible, each session ($dbh) is independent from the trans-
actions in other sessions. This is useful when you need to hold
cursors open across transactions--for example, if you use one ses-
sion for your long lifespan cursors (typically read-only) and
another for your short update transactions.
For compatibility with old DBI scripts, the driver can be specified
by passing its name as the fourth argument to "connect" (instead of
"\%attr"):
$dbh = DBI->connect($data_source, $user, $pass, $driver);
In this "old-style" form of "connect", the $data_source should not
start with ""dbi:driver_name:"". (If it does, the embedded
driver_name will be ignored). Also note that in this older form of
"connect", the "$dbh-">"{AutoCommit}" attribute is undefined, the
"$dbh-">"{PrintError}" attribute is off, and the old "DBI_DBNAME"
environment variable is checked if "DBI_DSN" is not defined. Beware
that this "old-style" "connect" will be withdrawn in a future ver-
sion of DBI.
"connect_cached" NEW
$dbh = DBI->connect_cached($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
or die $DBI::errstr;
"connect_cached" is like "connect", except that the database handle
returned is also stored in a hash associated with the given parame-
ters. If another call is made to "connect_cached" with the same
parameter values, then the corresponding cached $dbh will be
returned if it is still valid. The cached database handle is
replaced with a new connection if it has been disconnected or if
the "ping" method fails.
Note that the behavior of this method differs in several respects
from the behavior of presistent connections implemented by
Apache::DBI.
Caching can be useful in some applications, but it can also cause
problems and should be used with care. The exact behavior of this
method is liable to change, so if you intend to use it in any pro-
duction applications you should discuss your needs on the dbi-users
mailing list.
The cache can be accessed (and cleared) via the "CachedKids"
attribute.
"available_drivers"
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($quiet);
Returns a list of all available drivers by searching for "DBD::*"
modules through the directories in @INC. By default, a warning is
given if some drivers are hidden by others of the same name in ear-
lier directories. Passing a true value for $quiet will inhibit the
warning.
"data_sources"
@ary = DBI->data_sources($driver);
@ary = DBI->data_sources($driver, \%attr);
Returns a list of all data sources (databases) available via the
named driver. If $driver is empty or "undef", then the value of
the "DBI_DRIVER" environment variable is used.
The driver will be loaded if it hasn't been already. Note that if
the driver loading fails then it dies with an error message that
includes the string ""install_driver"" and the underlying problem.
Data sources are returned in a form suitable for passing to the
"connect" method (that is, they will include the ""dbi:$driver:""
prefix).
Note that many drivers have no way of knowing what data sources
might be available for it. These drivers return an empty or incom-
plete list or may require driver-specific attributes, such as a
connected database handle, to be supplied.
"trace"
DBI->trace($trace_level)
DBI->trace($trace_level, $trace_filename)
DBI trace information can be enabled for all handles using the
"trace" DBI class method. To enable trace information for a spe-
cific handle, use the similar "$h-">"trace" method described else-
where.
Trace levels are as follows:
0 - Trace disabled.
1 - Trace DBI method calls returning with results or errors.
2 - Trace method entry with parameters and returning with results.
3 - As above, adding some high-level information from the driver
and some internal information from the DBI.
4 - As above, adding more detailed information from the driver.
5 and above - As above but with more and more obscure information.
Trace level 1 is best for a simple overview of what's happening.
Trace level 2 is a good choice for general purpose tracing. Levels
3 and above (up to 9) are best reserved for investigating a spe-
cific problem, when you need to see "inside" the driver and DBI.
The trace output is detailed and typically very useful. Much of the
trace output is formatted using the "neat" function, so strings in
the trace output may be edited and truncated.
Initially trace output is written to "STDERR". If $trace_filename
is specified and can be opened in append mode then all trace output
(including that from other handles) is redirected to that file. A
warning is generated is the file can't be opened. Further calls to
"trace" without a $trace_filename do not alter where the trace
output is sent. If $trace_filename is undefined, then trace output
is sent to "STDERR" and the previous trace file is closed. The
"trace" method returns the previous tracelevel.
See also the "$h-">"trace" and "$h-">"trace_msg" methods and the
"DEBUGGING" section for information about the "DBI_TRACE" environ-
ment variable.
DBI Utility Functions
In addition to the methods listed in the previous section, the DBI
package also provides these utility functions:
"neat"
$str = DBI::neat($value, $maxlen);
Return a string containing a neat (and tidy) representation of the
supplied value.
Strings will be quoted, although internal quotes will not be
escaped. Values known to be numeric will be unquoted. Undefined
(NULL) values will be shown as "undef" (without quotes). Unprint-
able characters will be replaced by dot (.).
For result strings longer than $maxlen the result string will be
truncated to "$maxlen-4" and ""...'"" will be appended. If $maxlen
is 0 or "undef", it defaults to $DBI::neat_maxlen which, in turn,
defaults to 400.
This function is designed to format values for human consumption.
It is used internally by the DBI for "trace" output. It should typ-
ically not be used for formatting values for database use. (See
also "quote".)
"neat_list"
$str = DBI::neat_list(\@listref, $maxlen, $field_sep);
Calls "DBI::neat" on each element of the list and returns a string
containing the results joined with $field_sep. $field_sep defaults
to ", ".
"looks_like_number"
@bool = DBI::looks_like_number(@array);
Returns true for each element that looks like a number. Returns
false for each element that does not look like a number. Returns
"undef" for each element that is undefined or empty.
"hash"
$hash_value = DBI::hash($buffer, $type);
Return a 32-bit integer 'hash' value corresponding to the contents
of $buffer. The $type parameter selects which kind of hash algo-
rithm should be used.
For the technically curious, type 0 (which is the default if $type
isn't specified) is based on the Perl 5.1 hash except that the
value is forced to be negative (for obscure historical reasons).
Type 1 is the better "Fowler / Noll / Vo" (FNV) hash. See
http://www.isthe.com/chongo/tech/comp/fnv/ for more information.
Both types are implemented in C and are very fast.
This function doesn't have much to do with databases, except that
it can be handy to store hash values in a database.
DBI Dynamic Attributes
Dynamic attributes are always associated with the last handle used
(that handle is represented by $h in the descriptions below).
Where an attribute is equivalent to a method call, then refer to the
method call for all related documentation.
Warning: these attributes are provided as a convenience but they do
have limitations. Specifically, they have a short lifespan: because
they are associated with the last handle used, they should only be used
immediately after calling the method that "sets" them. If in any
doubt, use the corresponding method call.
$DBI::err
Equivalent to "$h-">"err".
$DBI::errstr
Equivalent to "$h-">"errstr".
$DBI::state
Equivalent to "$h-">"state".
$DBI::rows
Equivalent to "$h-">"rows". Please refer to the documentation for
the "rows" method.
$DBI::lasth
Returns the DBI object handle used for the most recent DBI method
call. If the last DBI method call was a DESTROY then $DBI::lasth
will return the handle of the parent of the destroyed handle, if
there is one.
METHODS COMMON TO ALL HANDLES
The following methods can be used by all types of DBI handles.
"err"
$rv = $h->err;
Returns the native database engine error code from the last driver
method called. The code is typically an integer but you should not
assume that.
The DBI resets $h->err to undef before most DBI method calls, so
the value only has a short lifespan. Also, most drivers share the
same error variables across all their handles, so calling a method
on one handle will typically reset the error on all the other han-
dles that are children of that driver.
If you need to test for individual errors and have your program be
portable to different database engines, then you'll need to deter-
mine what the corresponding error codes are for all those engines
and test for all of them.
"errstr"
$str = $h->errstr;
Returns the native database engine error message from the last
driver method called. This has the same lifespan issues as the
"err" method described above.
"state"
$str = $h->state;
Returns an error code in the standard SQLSTATE five character for-
mat. Note that the specific success code 00000 is translated to ''
(false). If the driver does not support SQLSTATE (and most don't),
then state will return "S1000" (General Error) for all errors.
The driver is free to return any value via "state", e.g., warning
codes, even if it has not declared an error by returning a true
value via the "err" method described above.
"set_err" NEW
$rv = $h->set_err($err, $errstr);
$rv = $h->set_err($err, $errstr, $state, $method);
$rv = $h->set_err($err, $errstr, $state, $method, $rv);
Set the "err", "errstr", and "state" values for the handle. This
will trigger the normal DBI error handling mechanisms, such as
"RaiseError" and "HandleError", if they are enabled. This method
is typically only used by DBI drivers and DBI subclasses.
The $method parameter provides an alternate method name, instead of
the fairly unhelpful '"set_err"', for the "RaiseError"/"PrintError"
error string.
The "set_err" method normally returns undef. The $rv parameter
provides an alternate return value. The "HandleError" subroutine
can access and alter this value.
"trace"
$h->trace($trace_level);
$h->trace($trace_level, $trace_filename);
DBI trace information can be enabled for a specific handle (and any
future children of that handle) by setting the trace level using
the "trace" method.
Trace level 1 is best for a simple overview of what's happening.
Trace level 2 is a good choice for general purpose tracing. Levels
3 and above (up to 9) are best reserved for investigating a spe-
cific problem, when you need to see "inside" the driver and DBI.
Set $trace_level to 0 to disable the trace.
The trace output is detailed and typically very useful. Much of the
trace output is formatted using the "neat" function, so strings in
the trace output may be edited and truncated.
Initially, trace output is written to "STDERR". If $trace_filename
is specified, then the file is opened in append mode and all trace
output (including that from other handles) is redirected to that
file. Further calls to trace without a $trace_filename do not
alter where the trace output is sent. If $trace_filename is unde-
fined, then trace output is sent to "STDERR" and the previous trace
file is closed.
See also the "DBI-">"trace" method, the "$h-">"{TraceLevel}"
attribute, and "DEBUGGING" for information about the "DBI_TRACE"
environment variable.
"trace_msg"
$h->trace_msg($message_text);
$h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if trace is enabled for $h
or for the DBI as a whole. Can also be called as
"DBI-">"trace_msg($msg)". See "trace".
If $min_level is defined, then the message is output only if the
trace level is equal to or greater than that level. $min_level
defaults to 1.
"func"
$h->func(@func_arguments, $func_name) or die ...;
The "func" method can be used to call private non-standard and non-
portable methods implemented by the driver. Note that the function
name is given as the last argument.
It's also important to note that the func() method does not clear a
previous error ($DBI::err etc.) and it does not trigger automatic
error detection (RaiseError etc.) so you must check the return sta-
tus and/or $h->err to detect errors.
(This method is not directly related to calling stored procedures.
Calling stored procedures is currently not defined by the DBI.
Some drivers, such as DBD::Oracle, support it in non-portable ways.
See driver documentation for more details.)
ATTRIBUTES COMMON TO ALL HANDLES
These attributes are common to all types of DBI handles.
Some attributes are inherited by child handles. That is, the value of
an inherited attribute in a newly created statement handle is the same
as the value in the parent database handle. Changes to attributes in
the new statement handle do not affect the parent database handle and
changes to the database handle do not affect existing statement han-
dles, only future ones.
Attempting to set or get the value of an unknown attribute is fatal,
except for private driver specific attributes (which all have names
starting with a lowercase letter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
"Warn" (boolean, inherited)
Enables useful warnings for certain bad practices. Enabled by
default. Some emulation layers, especially those for Perl 4 inter-
faces, disable warnings. Since warnings are generated using the
Perl "warn" function, they can be intercepted using the Perl
$SIG{__WARN__} hook.
"Active" (boolean, read-only)
True if the handle object is "active". This is rarely used in
applications. The exact meaning of active is somewhat vague at the
moment. For a database handle it typically means that the handle is
connected to a database ("$dbh-">"disconnect" sets "Active" off).
For a statement handle it typically means that the handle is a
"SELECT" that may have more data to fetch. (Fetching all the data
or calling "$sth-">"finish" sets "Active" off.)
"Kids" (integer, read-only)
For a driver handle, "Kids" is the number of currently existing
database handles that were created from that driver handle. For a
database handle, "Kids" is the number of currently existing state-
ment handles that were created from that database handle.
"ActiveKids" (integer, read-only)
Like "Kids", but only counting those that are "Active" (as above).
"CachedKids" (hash ref)
For a database handle, returns a reference to the cache (hash) of
statement handles created by the "prepare_cached" method. For a
driver handle, returns a reference to the cache (hash) of database
handles created by the "connect_cached" method.
"CompatMode" (boolean, inherited)
Used by emulation layers (such as Oraperl) to enable compatible
behavior in the underlying driver (e.g., DBD::Oracle) for this han-
dle. Not normally set by application code.
"InactiveDestroy" (boolean)
This attribute can be used to disable the database engine related
effect of DESTROYing a handle (which would normally close a pre-
pared statement or disconnect from the database etc).
For a database handle, this attribute does not disable an explicit
call to the disconnect method, only the implicit call from DESTROY.
The default value, false, means that a handle will be automatically
destroyed when it passes out of scope. A true value disables auto-
matic destruction. (Think of the name as meaning 'inactive the
DESTROY method'.)
This attribute is specifically designed for use in Unix applica-
tions that "fork" child processes. Either the parent or the child
process, but not both, should set "InactiveDestroy" on all their
shared handles. Note that some databases, including Oracle, don't
support passing a database connection across a fork.
"PrintError" (boolean, inherited)
This attribute can be used to force errors to generate warnings
(using "warn") in addition to returning error codes in the normal
way. When set "on", any method which results in an error occuring
will cause the DBI to effectively do a "warn("$class $method
failed: $DBI::errstr")" where $class is the driver class and
$method is the name of the method which failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
By default, "DBI-">"connect" sets "PrintError" "on".
If desired, the warnings can be caught and processed using a
$SIG{__WARN__} handler or modules like CGI::Carp and CGI::Error-
Wrap.
"RaiseError" (boolean, inherited)
This attribute can be used to force errors to raise exceptions
rather than simply return error codes in the normal way. It is
"off" by default. When set "on", any method which results in an
error will cause the DBI to effectively do a "die("$class $method
failed: $DBI::errstr")", where $class is the driver class and
$method is the name of the method that failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If you turn "RaiseError" on then you'd normally turn "PrintError"
off. If "PrintError" is also on, then the "PrintError" is done
first (naturally).
Typically "RaiseError" is used in conjunction with "eval { ... }"
to catch the exception that's been thrown and followed by an "if
($@) { ... }" block to handle the caught exception. In that eval
block the $DBI::lasth variable can be useful for diagnosis and
reporting. For example, $DBI::lasth->{Type} and
$DBI::lasth->{Statement}.
If you want to temporarily turn "RaiseError" off (inside a library
function that is likely to fail, for example), the recommended way
is like this:
{
local $h->{RaiseError}; # localize and turn off for this block
...
}
The original value will automatically and reliably be restored by
Perl, regardless of how the block is exited. The same logic
applies to other attributes, including "PrintError".
Sadly, this doesn't work for Perl versions up to and including
5.004_04. Even more sadly, for Perl 5.5 and 5.6.0 it does work but
leaks memory! For backwards compatibility, you could just use
"eval { ... }" instead.
"HandleError" (code ref, inherited)
This attribute can be used to provide your own alternative
behaviour in case of errors. If set to a reference to a subroutine
then that subroutine is called when an error is detected (at the
same point that "RaiseError" and "PrintError" are handled).
The subroutine is called with three parameters: the error message
string that "RaiseError" and "PrintError" would use, the DBI handle
being used, and the first value being returned by the method that
failed (typically undef).
If the subroutine returns a false value then the "RaiseError"
and/or "PrintError" attributes are checked and acted upon as nor-
mal.
For example, to "die" with a full stack trace for any error:
use Carp;
$h->{HandleError} = sub { confess(shift) };
Or to turn errors into exceptions:
use Exception; # or your own favourite exception module
$h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };
It is possible to 'stack' multiple HandleError handlers by using
closures:
sub your_subroutine {
my $previous_handler = $h->{HandleError};
$h->{HandleError} = sub {
return 1 if $previous_handler and &$previous_handler(@_);
... your code here ...
};
}
Using a "my" inside a subroutine to store the previous "HandleEr-
ror" value is important. See perlsub and perlref for more informa-
tion about closures.
It is possible for "HandleError" to alter the error message that
will be used by "RaiseError" and "PrintError" if it returns false.
It can do that by altering the value of $_[0]. This example appends
a stack trace to all errors and, unlike the previous example using
Carp::confess, this will work "PrintError" as well as "RaiseError":
$h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };
It is also possible for "HandleError" to hide an error, to a lim-
ited degree, by using "set_err" to reset $DBI::err and
$DBI::errstr, and altering the return value of the failed method.
For example:
$h->{HandleError} = sub {
return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
return 0 unless $_[1]->err == 1234; # the error to 'hide'
$h->set_err(0,""); # turn off the error
$_[2] = [ ... ]; # supply alternative return value
return 1;
};
This only works for methods which return a single value and is hard
to make reliable (avoiding infinite loops, for example) and so
isn't recommended for general use! If you find a good use for it
then please let me know.
"ShowErrorStatement" (boolean, inherited) NEW
This attribute can be used to cause the relevant Statement text to
be appended to the error messages generated by the "RaiseError" and
"PrintError" attributes. Only applies to errors on statement han-
dles plus the prepare(), do(), and the various "select*()" database
handle methods. (The exact format of the appended text is subject
to change.)
If "$h->{ParamValues}" returns a hash reference of parameter
(placeholder) values then those are formatted and appened to the
end of the Statement text in the error message.
"TraceLevel" (integer, inherited) NEW
This attribute can be used as an alternative to the "trace" method
to set the DBI trace level for a specific handle.
"FetchHashKeyName" (string, inherited)
This attribute is used to specify whether the fetchrow_hashref()
method should perform case conversion on the field names used for
the hash keys. For historical reasons it defaults to '"NAME"' but
it is recommended to set it to '"NAME_lc"' (convert to lower case)
or '"NAME_uc"' (convert to upper case) according to your prefer-
ence. It can only be set for driver and database handles. For
statement handles the value is frozen when prepare() is called.
"ChopBlanks" (boolean, inherited)
This attribute can be used to control the trimming of trailing
space characters from fixed width character (CHAR) fields. No other
field types are affected, even where field values have trailing
spaces.
The default is false (although it is possible that the default may
change). Applications that need specific behavior should set the
attribute as needed. Emulation interfaces should set the attribute
to match the behavior of the interface they are emulating.
Drivers are not required to support this attribute, but any driver
which does not support it must arrange to return "undef" as the
attribute value.
"LongReadLen" (unsigned integer, inherited)
This attribute may be used to control the maximum length of long
fields ("blob", "memo", etc.) which the driver will read from the
database automatically when it fetches each row of data. The "Lon-
gReadLen" attribute only relates to fetching and reading long val-
ues; it is not involved in inserting or updating them.
A value of 0 means not to automatically fetch any long data.
("fetch" should return "undef" for long fields when "LongReadLen"
is 0.)
The default is typically 0 (zero) bytes but may vary between
drivers. Applications fetching long fields should set this value
to slightly larger than the longest long field value to be fetched.
Some databases return some long types encoded as pairs of hex dig-
its. For these types, "LongReadLen" relates to the underlying data
length and not the doubled-up length of the encoded string.
Changing the value of "LongReadLen" for a statement handle after it
has been "prepare"'d will typically have no effect, so it's common
to set "LongReadLen" on the $dbh before calling "prepare".
Note that the value used here has a direct effect on the memory
used by the application, so don't be too generous.
See "LongTruncOk" for more information on truncation behavior.
"LongTruncOk" (boolean, inherited)
This attribute may be used to control the effect of fetching a long
field value which has been truncated (typically because it's longer
than the value of the "LongReadLen" attribute).
By default, "LongTruncOk" is false and so fetching a long value
that needs to be truncated will cause the fetch to fail. (Applica-
tions should always be sure to check for errors after a fetch loop
in case an error, such as a divide by zero or long field trunca-
tion, caused the fetch to terminate prematurely.)
If a fetch fails due to a long field truncation when "LongTruncOk"
is false, many drivers will allow you to continue fetching further
rows.
See also "LongReadLen".
"Taint" (boolean, inherited)
If this attribute is set to a true value and Perl is running in
taint mode (e.g., started with the "-T" option), then all the argu-
ments to most DBI method calls are checked for being tainted. This
may change.
The attribute defaults to off, even if Perl is in taint mode. See
perlsec for more about taint mode. If Perl is not running in taint
mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintIn
attribute, for that statement handle, for the duration of the fetch
loop.
"TaintOut" (boolean, inherited)
If this attribute is set to a true value and Perl is running in
taint mode (e.g., started with the "-T" option), then most data
fetched from the database is considered tainted. This may change.
The attribute defaults to off, even if Perl is in taint mode. See
perlsec for more about taint mode. If Perl is not running in taint
mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintOut
attribute, for that statement handle, for the duration of the fetch
loop.
Currently only fetched data is tainted. It is possible that the
results of other DBI method calls, and the value of fetched
attributes, may also be tainted in future versions. That change may
well break your applications unless you take great care now. If you
use DBI Taint mode, please report your experience and any sugges-
tions for changes.
"Taint" (boolean, inherited)
This value is shortcut for "TaintIn" and "TaintOut" (it is also
present for backwards compatability).
Setting this attribute sets both "TaintIn" and "TaintOut", and
retrieving it returns a true value if and only if "TaintIn" and
"TaintOut" are both set to true values.
"Profile" (inherited)
Enable collection and reporting of method call timing statistics.
See the DBI::Profile module documentation for much more detail.
"private_your_module_name_*"
The DBI provides a way to store extra information in a DBI handle
as "private" attributes. The DBI will allow you to store and
retreive any attribute which has a name starting with ""private_"".
It is strongly recommended that you use just one private attribute
(e.g., use a hash ref) and give it a long and unambiguous name that
includes the module or application name that the attribute relates
to (e.g., ""private_YourFullModuleName_thingy"").
Because of the way the Perl tie mechanism works you cannot reliably
use the "||=" operator directly to initialise the attribute, like
this:
my $foo = $dbh->{private_yourmodname_foo} ||= { ... }; # WRONG
you should use a two step approach like this:
my $foo = $dbh->{private_yourmodname_foo};
$foo ||= $dbh->{private_yourmodname_foo} = { ... };
DBI DATABASE HANDLE OBJECTS
This section covers the methods and attributes associated with database
handles.
Database Handle Methods
The following methods are specified for DBI database handles:
"do"
$rows = $dbh->do($statement) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr, @bind_values) or die ...
Prepare and execute a single statement. Returns the number of rows
affected or "undef" on error. A return value of "-1" means the num-
ber of rows is not known, not applicable, or not available.
This method is typically most useful for non-"SELECT" statements
that either cannot be prepared in advance (due to a limitation of
the driver) or do not need to be executed repeatedly. It should not
be used for "SELECT" statements because it does not return a state-
ment handle (so you can't fetch any data).
The default "do" method is logically similar to:
sub do {
my($dbh, $statement, $attr, @bind_values) = @_;
my $sth = $dbh->prepare($statement, $attr) or return undef;
$sth->execute(@bind_values) or return undef;
my $rows = $sth->rows;
($rows == 0) ? "0E0" : $rows; # always return true if no error
}
For example:
my $rows_deleted = $dbh->do(q{
DELETE FROM table
WHERE status = ?
}, undef, 'DONE') or die $dbh->errstr;
Using placeholders and @bind_values with the "do" method can be
useful because it avoids the need to correctly quote any variables
in the $statement. But if you'll be executing the statement many
times then it's more efficient to "prepare" it once and call "exe-
cute" many times instead.
The "q{...}" style quoting used in this example avoids clashing
with quotes that may be used in the SQL statement. Use the double-
quote-like "qq{...}" operator if you want to interpolate variables
into the string. See "Quote and Quote-like Operators" in perlop
for more details.
"selectrow_array"
@row_ary = $dbh->selectrow_array($statement);
@row_ary = $dbh->selectrow_array($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_array" into a single call. If called in a list context,
it returns the first row of data from the statement. The $state-
ment parameter can be a previously prepared statement handle, in
which case the "prepare" is skipped.
If any method fails, and "RaiseError" is not set, "selectrow_array"
will return an empty list.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return the
value of the first column or the last. So don't do that. Also, in
a scalar context, an "undef" is returned if there are no more rows
or if an error occurred. That "undef" can't be distinguished from
an "undef" returned because the first field value was NULL. For
these reasons you should exercise some caution if you use "selec-
trow_array" in a scalar context.
"selectrow_arrayref"
$ary_ref = $dbh->selectrow_arrayref($statement);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_arrayref" into a single call. It returns the first row of
data from the statement. The $statement parameter can be a previ-
ously prepared statement handle, in which case the "prepare" is
skipped.
If any method fails, and "RaiseError" is not set, "selectrow_array"
will return undef.
"selectrow_hashref"
$hash_ref = $dbh->selectrow_hashref($statement);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_hashref" into a single call. It returns the first row of
data from the statement. The $statement parameter can be a previ-
ously prepared statement handle, in which case the "prepare" is
skipped.
If any method fails, and "RaiseError" is not set, "selec-
trow_hashref" will return undef.
"selectall_arrayref"
$ary_ref = $dbh->selectall_arrayref($statement);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchall_arrayref" into a single call. It returns a reference to
an array containing a reference to an array for each row of data
fetched.
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If "RaiseError" is not set and any method except
"fetchall_arrayref" fails then "selectall_arrayref" will return
"undef"; if "fetchall_arrayref" fails then it will return with
whatever data has been fetched thus far. You should check
"$sth-">"err" afterwards (or use the "RaiseError" attribute) to
discover if the data is complete or was truncated due to an error.
The "fetchall_arrayref" method called by "selectall_arrayref" sup-
ports a $max_rows parameter. You can specify a value for $max_rows
by including a '"MaxRows"' attribute in \%attr.
The "fetchall_arrayref" method called by "selectall_arrayref" also
supports a $slice parameter. You can specify a value for $slice by
including a '"Slice"' or '"Columns"' attribute in \%attr. The only
difference between the two is that if "Slice" is not defined and
"Columns" is an array ref, then the array is assumed to contain
column index values (which count from 1), rather than perl array
index values. In which case the array is copied and each value
decremented before passing to "/fetchall_arrayref".
"selectall_hashref"
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchall_hashref" into a single call. It returns a reference to a
hash containing one entry for each row. The key for each row entry
is specified by $key_field. The value is a reference to a hash
returned by "fetchrow_hashref".
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If any method except "fetchrow_hashref" fails, and "RaiseError" is
not set, "selectall_hashref" will return "undef". If
"fetchrow_hashref" fails and "RaiseError" is not set, then it will
return with whatever data it has fetched thus far. $DBI::err should
be checked to catch that.
"selectcol_arrayref"
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute", and fetching one
column from all the rows, into a single call. It returns a refer-
ence to an array containing the values of the first column from
each row.
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If any method except "fetch" fails, and "RaiseError" is not set,
"selectcol_arrayref" will return "undef". If "fetch" fails and
"RaiseError" is not set, then it will return with whatever data it
has fetched thus far. $DBI::err should be checked to catch that.
The "selectcol_arrayref" method defaults to pushing a single column
value (the first) from each row into the result array. However, it
can also push another column, or even multiple columns per row,
into the result array. This behaviour can be specified via a
'"Columns"' attribute which must be a ref to an array containing
the column number or numbers to use. For example:
# get array of id and name pairs:
my $ary_ref = $dbh->selectcol_arrayref("select id, name from table", { Columns=>[1,2] });
my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name
"prepare"
$sth = $dbh->prepare($statement) or die $dbh->errstr;
$sth = $dbh->prepare($statement, \%attr) or die $dbh->errstr;
Prepares a single statement for later execution by the database
engine and returns a reference to a statement handle object.
The returned statement handle can be used to get attributes of the
statement and invoke the "execute" method. See "Statement Handle
Methods".
Drivers for engines without the concept of preparing a statement
will typically just store the statement in the returned handle and
process it when "$sth-">"execute" is called. Such drivers are
unlikely to give much useful information about the statement, such
as "$sth-">"{NUM_OF_FIELDS}", until after "$sth-">"execute" has
been called. Portable applications should take this into account.
In general, DBI drivers do not parse the contents of the statement
(other than simply counting any "Placeholders"). The statement is
passed directly to the database engine, sometimes known as pass-
thru mode. This has advantages and disadvantages. On the plus side,
you can access all the functionality of the engine being used. On
the downside, you're limited if you're using a simple engine, and
you need to take extra care if writing applications intended to be
portable between engines.
Portable applications should not assume that a new statement can be
prepared and/or executed while still fetching results from a previ-
ous statement.
Some command-line SQL tools use statement terminators, like a semi-
colon, to indicate the end of a statement. Such terminators should
not normally be used with the DBI.
"prepare_cached"
$sth = $dbh->prepare_cached($statement)
$sth = $dbh->prepare_cached($statement, \%attr)
$sth = $dbh->prepare_cached($statement, \%attr, $allow_active)
Like "prepare" except that the statement handle returned will be
stored in a hash associated with the $dbh. If another call is made
to "prepare_cached" with the same $statement and %attr values, then
the corresponding cached $sth will be returned without contacting
the database server.
Here are some examples of "prepare_cached":
sub insert_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $sql = sprintf "insert into %s (%s) values (%s)",
$table, join(",", @fields), join(",", ("?")x@fields);
my $sth = $dbh->prepare_cached($sql);
return $sth->execute(@values);
}
sub search_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $qualifier = "";
$qualifier = "where ".join(" and ", map { "$_=?" } @fields) if @fields;
$sth = $dbh->prepare_cached("SELECT * FROM $table $qualifier");
return $dbh->selectall_arrayref($sth, {}, @values);
}
Caveat emptor: This caching can be useful in some applications, but
it can also cause problems and should be used with care. Here is a
contrived case where caching would cause a significant problem:
my $sth = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth->execute($bar);
while (my $data = $sth->fetchrow_hashref) {
my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth2->execute($data->{bar});
while (my $data2 = $sth2->fetchrow_arrayref) {
do_stuff(...);
}
}
In this example, since both handles are preparing the exact same
statement, $sth2 will not be its own statement handle, but a dupli-
cate of $sth returned from the cache. The results will certainly
not be what you expect. Typically the the inner fetch loop will
work normally, fetching all the records and terminating when there
are no more, but now $sth is the same as $sth2 the outer fetch loop
will also terminate.
The $allow_active parameter lets you adjust DBI's behavior when
prepare_cached is returning a statement handle that is still
active. There are three settings:
0: A warning will be generated, and "finish" will be called on
the statement handle before it is returned. This is the
default behavior if $allow_active is not passed.
1: "finish" will be called on the statement handle, but the
warning is suppressed.
2: DBI will not touch the statement handle before returning it.
You will need to check "$sth-">"{Active}" on the returned
statement handle and deal with it in your own code.
Because the cache used by prepare_cached() is keyed by all the
parameters, including any attributes passed, you can also avoid
this issue by doing something like:
my $sth = $dbh->prepare_cached("...", { dbi_dummy => __FILE__.__LINE__ });
which will ensure that prepare_cached only returns statements
cached by that line of code in that source file.
"commit"
$rc = $dbh->commit or die $dbh->errstr;
Commit (make permanent) the most recent series of database changes
if the database supports transactions and AutoCommit is off.
If "AutoCommit" is on, then calling "commit" will issue a "commit
ineffective with AutoCommit" warning.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"rollback"
$rc = $dbh->rollback or die $dbh->errstr;
Rollback (undo) the most recent series of uncommitted database
changes if the database supports transactions and AutoCommit is
off.
If "AutoCommit" is on, then calling "rollback" will issue a "roll-
back ineffective with AutoCommit" warning.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"begin_work"
$rc = $dbh->begin_work or die $dbh->errstr;
Enable transactions (by turning "AutoCommit" off) until the next
call to "commit" or "rollback". After the next "commit" or "roll-
back", "AutoCommit" will automatically be turned on again.
If "AutoCommit" is already off when "begin_work" is called then it
does nothing except return an error. If the driver does not support
transactions then when "begin_work" attempts to set "AutoCommit"
off the driver will trigger a fatal error.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"disconnect"
$rc = $dbh->disconnect or warn $dbh->errstr;
Disconnects the database from the database handle. "disconnect" is
typically only used before exiting the program. The handle is of
little use after disconnecting.
The transaction behavior of the "disconnect" method is, sadly,
undefined. Some database systems (such as Oracle and Ingres) will
automatically commit any outstanding changes, but others (such as
Informix) will rollback any outstanding changes. Applications not
using "AutoCommit" should explicitly call "commit" or "rollback"
before calling "disconnect".
The database is automatically disconnected by the "DESTROY" method
if still connected when there are no longer any references to the
handle. The "DESTROY" method for each driver should implicitly
call "rollback" to undo any uncommitted changes. This is vital
behavior to ensure that incomplete transactions don't get committed
simply because Perl calls "DESTROY" on every object before exiting.
Also, do not rely on the order of object destruction during "global
destruction", as it is undefined.
Generally, if you want your changes to be commited or rolled back
when you disconnect, then you should explicitly call "commit" or
"rollback" before disconnecting.
If you disconnect from a database while you still have active
statement handles (e.g., SELECT statement handles that may have
more data to fetch), you will get a warning. The warning may indi-
cate that a fetch loop terminated early, perhaps due to an uncaught
error. To avoid the warning call the "finish" method on the active
handles.
"ping"
$rc = $dbh->ping;
Attempts to determine, in a reasonably efficient way, if the
database server is still running and the connection to it is still
working. Individual drivers should implement this function in the
most suitable manner for their database engine.
The current default implementation always returns true without
actually doing anything. Actually, it returns ""0 but true"" which
is true but zero. That way you can tell if the return value is gen-
uine or just the default. Drivers should override this method with
one that does the right thing for their type of database.
Few applications would have direct use for this method. See the
specialized Apache::DBI module for one example usage.
"get_info" NEW
Warning: This method is experimental and may change.
$value = $dbh->get_info( $info_type );
Returns information about the implementation, i.e. driver and data
source capabilities, restrictions etc. It returns "undef" for
unknown or unimplemented information types. For example:
$database_version = $dbh->get_info( 18 ); # SQL_DBMS_VER
$max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT
See "Standards Reference Information" for more detailed information
about the information types and their meanings and possible return
values.
The DBI curently doesn't provide a name to number mapping for the
information type codes or the results. Applications are expected to
use the integer values directly, with the name in a comment, or
define their own named values using something like the constant
pragma.
Because some DBI methods make use of get_info(), drivers are
strongly encouraged to support at least the following very minimal
set of information types to ensure the DBI itself works properly:
Type Name Example A Example B
---- -------------------------- ------------ ------------
17 SQL_DBMS_NAME 'ACCESS' 'Oracle'
18 SQL_DBMS_VER '03.50.0000' '08.01.0721'
29 SQL_IDENTIFIER_QUOTE_CHAR '`' '"'
41 SQL_CATALOG_NAME_SEPARATOR '.' '@'
114 SQL_CATALOG_LOCATION 1 2
"table_info" NEW
Warning: This method is experimental and may change.
$sth = $dbh->table_info( $catalog, $schema, $table, $type );
$sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );
$sth = $dbh->table_info( \%attr ); # old style
Returns an active statement handle that can be used to fetch infor-
mation about tables and views that exist in the database.
The old style interface passes all the parameters as a reference to
an attribute hash with some or all of the following attributes:
%attr = (
TABLE_CAT => $catalog # String value of the catalog name
, TABLE_SCHEM => $schema # String value of the schema name
, TABLE_NAME => $table # String value of the table name
, TABLE_TYPE => $type # String value of the table type(s)
);
The old style interface is deprecated and will be removed in a
future version.
The support for the selection criteria is driver specific. If the
driver doesn't support one or more of them then you may get back
more than you asked for and can do the filtering yourself.
The arguments $catalog, $schema and $table may accept search pat-
terns according to the database/driver, for example: $table =
'%FOO%'; Remember that the underscore character ('"_"') is a search
pattern that means match any character, so 'FOO_%' is the same as
'FOO%' and 'FOO_BAR%' will match names like 'FOO1BAR'.
The value of $type is a comma-separated list of one or more types
of tables to be returned in the result set. Each value may option-
ally be quoted, e.g.:
$type = "TABLE";
$type = "'TABLE','VIEW'";
In addition the following special cases may also be supported by
some drivers:
o If the value of $catalog is '%' and $schema and $table name are
empty strings, the result set contains a list of catalog names. For
example:
$sth = $dbh->table_info('%', '', '');
o If the value of $schema is '%' and $catalog and $table are empty
strings, the result set contains a list of schema names.
o If the value of $type is '%' and $catalog, $schema, and $table
are all empty strings, the result set contains a list of table
types.
The statement handle returned has at least the following fields in
the order show below. Other fields, after these, may also be
present.
TABLE_CAT: Table catalog identifier. This field is NULL ("undef")
if not applicable to the data source, which is usually the case.
This field is empty if not applicable to the table.
TABLE_SCHEM: The name of the schema containing the TABLE_NAME
value. This field is NULL ("undef") if not applicable to data
source, and empty if not applicable to the table.
TABLE_NAME: Name of the table (or view, synonym, etc).
TABLE_TYPE: One of the following: "TABLE", "VIEW", "SYSTEM TABLE",
"GLOBAL TEMPORARY", "LOCAL TEMPORARY", "ALIAS", "SYNONYM" or a type
identifier that is specific to the data source.
REMARKS: A description of the table. May be NULL ("undef").
Note that "table_info" might not return records for all tables.
Applications can use any valid table regardless of whether it's
returned by "table_info".
See also "tables", "Catalog Methods" and "Standards Reference
Information".
"column_info" NEW
Warning: This method is experimental and may change.
$sth = $dbh->column_info( $catalog, $schema, $table, $column );
Returns an active statement handle that can be used to fetch infor-
mation about columns in specified tables.
The arguments $schema, $table and $column may accept search pat-
terns according to the database/driver, for example: $table =
'%FOO%';
Note: The support for the selection criteria is driver specific. If
the driver doesn't support one or more of them then you may get
back more than you asked for and can do the filtering yourself.
The statement handle returned has at least the following fields in
the order shown below. Other fields, after these, may also be
present.
TABLE_CAT: The catalog identifier. This field is NULL ("undef") if
not applicable to the data source, which is often the case. This
field is empty if not applicable to the table.
TABLE_SCHEM: The schema identifier. This field is NULL ("undef")
if not applicable to the data source, and empty if not applicable
to the table.
TABLE_NAME: The table identifier. Note: A driver may provide col-
umn metadata not only for base tables, but also for derived objects
like SYNONYMS etc.
COLUMN_NAME: The column identifier.
DATA_TYPE: The concise data type code.
TYPE_NAME: A data source dependent data type name.
COLUMN_SIZE: The column size. This is the maximum length in char-
acters for character data types, the number of digits or bits for
numeric data types or the length in the representation of temporal
types. See the relevant specifications for detailed information.
BUFFER_LENGTH: The length in bytes of transferred data.
DECIMAL_DIGITS: The total number of significant digits to the right
of the decimal point.
NUM_PREC_RADIX: The radix for numeric precision. The value is 10
or 2 for numeric data types and NULL ("undef") if not applicable.
NULLABLE: Indicates if a column can accept NULLs. The following
values are defined:
SQL_NO_NULLS 0
SQL_NULLABLE 1
SQL_NULLABLE_UNKNOWN 2
REMARKS: A description of the column.
COLUMN_DEF: The default value of the column.
SQL_DATA_TYPE: The SQL data type.
SQL_DATETIME_SUB: The subtype code for datetime and interval data
types.
CHAR_OCTET_LENGTH: The maximum length in bytes of a character or
binary data type column.
ORDINAL_POSITION: The column sequence number (starting with 1).
IS_NULLABLE: Indicates if the column can accept NULLs. Possible
values are: 'NO', 'YES' and ''.
SQL/CLI defines the following additional columns:
CHAR_SET_CAT
CHAR_SET_SCHEM
CHAR_SET_NAME
COLLATION_CAT
COLLATION_SCHEM
COLLATION_NAME
UDT_CAT
UDT_SCHEM
UDT_NAME
DOMAIN_CAT
DOMAIN_SCHEM
DOMAIN_NAME
SCOPE_CAT
SCOPE_SCHEM
SCOPE_NAME
MAX_CARDINALITY
DTD_IDENTIFIER
IS_SELF_REF
Drivers capable of supplying any of those values should do so in
the corresponding column and supply undef values for the others.
Drivers wishing to provide extra database/driver specific informa-
tion should do so in extra columns beyond all those listed above,
and use lowercase field names with the driver-specific prefix
(i.e., 'ora_...'). Applications accessing such fields should do so
by name and not by column number.
The result set is ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME and
ORDINAL_POSITION.
Note: There is some overlap with statement attributes (in perl) and
SQLDescribeCol (in ODBC). However, SQLColumns provides more meta-
data.
See also "Catalog Methods" and "Standards Reference Information".
"primary_key_info" NEW
Warning: This method is experimental and may change.
$sth = $dbh->primary_key_info( $catalog, $schema, $table );
Returns an active statement handle that can be used to fetch infor-
mation about columns that make up the primary key for a table. The
arguments don't accept search patterns (unlike table_info()).
For example:
$sth = $dbh->primary_key_info( undef, $user, 'foo' );
$data = $sth->fetchall_arrayref;
Note: The support for the selection criteria, such as $catalog, is
driver specific. If the driver doesn't support catalogs and/or
schemas, it may ignore these criteria.
The statement handle returned has at least the following fields in
the order shown below. Other fields, after these, may also be
present.
TABLE_CAT: The catalog identifier. This field is NULL ("undef") if
not applicable to the data source, which is often the case. This
field is empty if not applicable to the table.
TABLE_SCHEM: The schema identifier. This field is NULL ("undef")
if not applicable to the data source, and empty if not applicable
to the table.
TABLE_NAME: The table identifier.
COLUMN_NAME: The column identifier.
KEY_SEQ: The column sequence number (starting with 1). Note: This
field is named ORDINAL_POSITION in SQL/CLI.
PK_NAME: The primary key constraint identifier. This field is NULL
("undef") if not applicable to the data source.
See also "Catalog Methods" and "Standards Reference Information".
"primary_key" NEW
Warning: This method is experimental and may change.
@key_column_names = $dbh->primary_key( $catalog, $schema, $table );
Simple interface to the primary_key_info() method. Returns a list
of the column names that comprise the primary key of the specified
table. The list is in primary key column sequence order.
"foreign_key_info" NEW
Warning: This method is experimental and may change.
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table );
Returns an active statement handle that can be used to fetch infor-
mation about foreign keys in and/or referencing the specified ta-
ble(s). The arguments don't accept search patterns (unlike ta-
ble_info()).
$pk_catalog, $pk_schema, $pk_table identify the primary (unique)
key table (PKT).
$fk_catalog, $fk_schema, $fk_table identify the foreign key table
(FKT).
If both PKT and FKT are given, the function returns the foreign
key, if any, in table FKT that refers to the primary (unique) key
of table PKT. (Note: In SQL/CLI, the result is implementa-
tion-defined.)
If only PKT is given, then the result set contains the primary key
of that table and all foreign keys that refer to it.
If only FKT is given, then the result set contains all foreign keys
in that table and the primary keys to which they refer. (Note: In
SQL/CLI, the result includes unique keys too.)
For example:
$sth = $dbh->foreign_key_info( undef, $user, 'master');
$sth = $dbh->foreign_key_info( undef, undef, undef , undef, $user, 'detail');
$sth = $dbh->foreign_key_info( undef, $user, 'master', undef, $user, 'detail');
Note: The support for the selection criteria, such as $catalog, is
driver specific. If the driver doesn't support catalogs and/or
schemas, it may ignore these criteria.
The statement handle returned has the following fields in the order
shown below. Because ODBC never includes unique keys, they define
different columns in the result set than SQL/CLI. SQL/CLI column
names are shown in parentheses.
PKTABLE_CAT ( UK_TABLE_CAT ): The primary (unique) key ta-
ble catalog identifier. This field is NULL ("undef") if not appli-
cable to the data source, which is often the case. This field is
empty if not applicable to the table.
PKTABLE_SCHEM ( UK_TABLE_SCHEM ): The primary (unique) key ta-
ble schema identifier. This field is NULL ("undef") if not appli-
cable to the data source, and empty if not applicable to the table.
PKTABLE_NAME ( UK_TABLE_NAME ): The primary (unique) key ta-
ble identifier.
PKCOLUMN_NAME (UK_COLUMN_NAME ): The primary (unique) key col-
umn identifier.
FKTABLE_CAT ( FK_TABLE_CAT ): The foreign key table catalog
identifier. This field is NULL ("undef") if not applicable to the
data source, which is often the case. This field is empty if not
applicable to the table.
FKTABLE_SCHEM ( FK_TABLE_SCHEM ): The foreign key table schema
identifier. This field is NULL ("undef") if not applicable to the
data source, and empty if not applicable to the table.
FKTABLE_NAME ( FK_TABLE_NAME ): The foreign key table identi-
fier.
FKCOLUMN_NAME ( FK_COLUMN_NAME ): The foreign key column iden-
tifier.
KEY_SEQ ( ORDINAL_POSITION ): The column sequence number
(starting with 1).
UPDATE_RULE ( UPDATE_RULE ): The referential action for
the UPDATE rule. The following codes are defined:
CASCADE 0
RESTRICT 1
SET NULL 2
NO ACTION 3
SET DEFAULT 4
DELETE_RULE ( DELETE_RULE ): The referential action for
the DELETE rule. The codes are the same as for UPDATE_RULE.
FK_NAME ( FK_NAME ): The foreign key name.
PK_NAME ( UK_NAME ): The primary (unique) key
name.
DEFERRABILITY ( DEFERABILITY ): The deferrability of the for-
eign key constraint. The following codes are defined:
INITIALLY DEFERRED 5
INITIALLY IMMEDIATE 6
NOT DEFERRABLE 7
( UNIQUE_OR_PRIMARY ): This column is necessary if a
driver includes all candidate (i.e. primary and alternate) keys in
the result set (as specified by SQL/CLI). The value of this column
is UNIQUE if the foreign key references an alternate key and PRI-
MARY if the foreign key references a primary key, or it may be
undefined if the driver doesn't have access to the information.
See also "Catalog Methods" and "Standards Reference Information".
"tables" NEW
Warning: This method is experimental and may change.
@names = $dbh->tables( $catalog, $schema, $table, $type );
@names = $dbh->tables; # deprecated
Simple interface to table_info(). Returns a list of matching table
names, possibly including a catalog/schema prefix.
See "table_info" for a description of the parameters.
If "$dbh-">"get_info(29)" returns true (29 is SQL_IDENTI-
FIER_QUOTE_CHAR) then the table names are constructed and quoted by
"quote_identifier" to ensure they are usable even if they contain
whitespace or reserved words etc.
"type_info_all"
Warning: This method is experimental and may change.
$type_info_all = $dbh->type_info_all;
Returns a reference to an array which holds information about each
data type variant supported by the database and driver. The array
and its contents should be treated as read-only.
The first item is a reference to an 'index' hash of "Name =">
"Index" pairs. The items following that are references to arrays,
one per supported data type variant. The leading index hash defines
the names and order of the fields within the arrays that follow it.
For example:
$type_info_all = [
{ TYPE_NAME => 0,
DATA_TYPE => 1,
COLUMN_SIZE => 2, # was PRECISION originally
LITERAL_PREFIX => 3,
LITERAL_SUFFIX => 4,
CREATE_PARAMS => 5,
NULLABLE => 6,
CASE_SENSITIVE => 7,
SEARCHABLE => 8,
UNSIGNED_ATTRIBUTE=> 9,
FIXED_PREC_SCALE => 10, # was MONEY originally
AUTO_UNIQUE_VALUE => 11, # was AUTO_INCREMENT originally
LOCAL_TYPE_NAME => 12,
MINIMUM_SCALE => 13,
MAXIMUM_SCALE => 14,
NUM_PREC_RADIX => 15,
},
[ 'VARCHAR', SQL_VARCHAR,
undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
],
[ 'INTEGER', SQL_INTEGER,
undef, "", "", undef,0, 0,1,0,0,0,undef,0, 0, 10
],
];
Note that more than one row may have the same value in the
"DATA_TYPE" field if there are different ways to spell the type
name and/or there are variants of the type with different
attributes (e.g., with and without "AUTO_UNIQUE_VALUE" set, with
and without "UNSIGNED_ATTRIBUTE", etc).
The rows are ordered by "DATA_TYPE" first and then by how closely
each type maps to the corresponding ODBC SQL data type, closest
first.
The meaning of the fields is described in the documentation for the
"type_info" method. The index values shown above (e.g., "NULLABLE
="> 6) are for illustration only. Drivers may define the fields
with a different order.
This method is not normally used directly. The "type_info" method
provides a more useful interface to the data.
Even though an 'index' hash is provided, all the field names in the
index hash defined above will always have the index values defined
above. This is defined behaviour so that you don't need to rely on
the index hash, which is handy because the lettercase of the keys
is not defined. It is usually uppercase, as show here, but drivers
are free to return names with any lettercase. Drivers are also free
to return extra driver-specific columns of information - though
it's recommended that they start at column index 50 to leave room
for expansion of the DBI/ODBC specification.
"type_info"
Warning: This method is experimental and may change.
@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information about one or
more variants of $data_type. The list is ordered by "DATA_TYPE"
first and then by how closely each type maps to the corresponding
ODBC SQL data type, closest first. If called in a scalar context
then only the first (best) element is returned.
If $data_type is undefined or "SQL_ALL_TYPES", then the list will
contain hashes for all data type variants supported by the database
and driver.
If $data_type is an array reference then "type_info" returns the
information for the first type in the array that has any matches.
The keys of the hash follow the same letter case conventions as the
rest of the DBI (see "Naming Conventions and Name Space"). The fol-
lowing items should exist:
TYPE_NAME (string)
Data type name for use in CREATE TABLE statements etc.
DATA_TYPE (integer)
SQL data type number.
COLUMN_SIZE (integer)
For numeric types, this is either the total number of digits
(if the NUM_PREC_RADIX value is 10) or the total number of bits
allowed in the column (if NUM_PREC_RADIX is 2).
For string types, this is the maximum size of the string in
bytes.
For date and interval types, this is the maximum number of
characters needed to display the value.
LITERAL_PREFIX (string)
Characters used to prefix a literal. A typical prefix is ""'""
for characters, or possibly ""0x"" for binary values passed as
hexadecimal. NULL ("undef") is returned for data types for
which this is not applicable.
LITERAL_SUFFIX (string)
Characters used to suffix a literal. Typically ""'"" for char-
acters. NULL ("undef") is returned for data types where this
is not applicable.
CREATE_PARAMS (string)
Parameter names for data type definition. For example, "CRE-
ATE_PARAMS" for a "DECIMAL" would be ""precision,scale"" if the
DECIMAL type should be declared as "DECIMAL("precision,scale")"
where precision and scale are integer values. For a "VARCHAR"
it would be ""max length"". NULL ("undef") is returned for
data types for which this is not applicable.
NULLABLE (integer)
Indicates whether the data type accepts a NULL value: 0 or an
empty string = no, 1 = yes, 2 = unknown.
CASE_SENSITIVE (boolean)
Indicates whether the data type is case sensitive in collations
and comparisons.
SEARCHABLE (integer)
Indicates how the data type can be used in a WHERE clause, as
follows:
0 - Cannot be used in a WHERE clause
1 - Only with a LIKE predicate
2 - All comparison operators except LIKE
3 - Can be used in a WHERE clause with any comparison operator
UNSIGNED_ATTRIBUTE (boolean)
Indicates whether the data type is unsigned. NULL ("undef") is
returned for data types for which this is not applicable.
FIXED_PREC_SCALE (boolean)
Indicates whether the data type always has the same precision
and scale (such as a money type). NULL ("undef") is returned
for data types for which this is not applicable.
AUTO_UNIQUE_VALUE (boolean)
Indicates whether a column of this data type is automatically
set to a unique value whenever a new row is inserted. NULL
("undef") is returned for data types for which this is not
applicable.
LOCAL_TYPE_NAME (string)
Localized version of the "TYPE_NAME" for use in dialog with
users. NULL ("undef") is returned if a localized name is not
available (in which case "TYPE_NAME" should be used).
MINIMUM_SCALE (integer)
The minimum scale of the data type. If a data type has a fixed
scale, then "MAXIMUM_SCALE" holds the same value. NULL
("undef") is returned for data types for which this is not
applicable.
MAXIMUM_SCALE (integer)
The maximum scale of the data type. If a data type has a fixed
scale, then "MINIMUM_SCALE" holds the same value. NULL
("undef") is returned for data types for which this is not
applicable.
SQL_DATA_TYPE (integer)
This column is the same as the "DATA_TYPE" column, except for
interval and datetime data types. For interval and datetime
data types, the "SQL_DATA_TYPE" field will return "SQL_INTER-
VAL" or "SQL_DATETIME", and the "SQL_DATETIME_SUB" field below
will return the subcode for the specific interval or datetime
data type. If this field is NULL, then the driver does not sup-
port or report on interval or date subtypes.
SQL_DATETIME_SUB (integer)
For interval or datetime data types, where the "SQL_DATA_TYPE"
field above is "SQL_INTERVAL" or "SQL_DATETIME", this field
will hold the subcode for the specific interval or datetime
data type. Otherwise it will be NULL ("undef").
NUM_PREC_RADIX (integer)
The radix value of the data type. For approximate numeric
types, "NUM_PREC_RADIX" contains the value 2 and "COLUMN_SIZE"
holds the number of bits. For exact numeric types,
"NUM_PREC_RADIX" contains the value 10 and "COLUMN_SIZE" holds
the number of decimal digits. NULL ("undef") is returned either
for data types for which this is not applicable or if the
driver cannot report this information.
INTERVAL_PRECISION (integer)
The interval leading precision for interval types. NULL is
returned either for data types for which this is not applicable
or if the driver cannot report this information.
For example, to find the type name for the fields in a select
statement you can do:
@names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{TYPE} }
Since DBI and ODBC drivers vary in how they map their types into
the ISO standard types you may need to search for more than one
type. Here's an example looking for a usable type to store a date:
$my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );
Similarly, to more reliably find a type to store small integers,
you could use a list starting with "SQL_SMALLINT", "SQL_INTEGER",
"SQL_DECIMAL", etc.
See also "Standards Reference Information".
"quote"
$sql = $dbh->quote($value);
$sql = $dbh->quote($value, $data_type);
Quote a string literal for use as a literal value in an SQL state-
ment, by escaping any special characters (such as quotation marks)
contained within the string and adding the required type of outer
quotation marks.
$sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
$dbh->quote("Don't");
For most database types, quote would return 'Don''t' (including the
outer quotation marks).
An undefined $value value will be returned as the string "NULL"
(without single quotation marks) to match how NULLs are represented
in SQL.
If $data_type is supplied, it is used to try to determine the
required quoting behavior by using the information returned by
"type_info". As a special case, the standard numeric types are
optimized to return $value without calling "type_info".
Quote will probably not be able to deal with all possible input
(such as binary data or data containing newlines), and is not
related in any way with escaping or quoting shell meta-characters.
There is no need to quote values being used with "Placeholders and
Bind Values".
"quote_identifier"
$sql = $dbh->quote_identifier( $name );
$sql = $dbh->quote_identifier( $name1, $name2, $name3, \%attr );
Quote an identifier (table name etc.) for use in an SQL statement,
by escaping any special characters (such as double quotation marks)
it contains and adding the required type of outer quotation marks.
Undefined names are ignored and the remainder are quoted and then
joined together, typically with a dot (".") character. For example:
$id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );
would, for most database types, return "Her schema"."My table"
(including all the double quotation marks).
If three names are supplied then the first is assumed to be a cata-
log name and special rules may be applied based on what "get_info"
returns for SQL_CATALOG_NAME_SEPARATOR (41) and SQL_CATALOG_LOCA-
TION (114). For example, for Oracle:
$id = $dbh->quote_identifier( 'link', 'schema', 'table' );
would return "schema"."table"@"link".
Database Handle Attributes
This section describes attributes specific to database handles.
Changes to these database handle attributes do not affect any other
existing or future database handles.
Attempting to set or get the value of an unknown attribute is fatal,
except for private driver-specific attributes (which all have names
starting with a lowercase letter).
Example:
$h->{AutoCommit} = ...; # set/write
... = $h->{AutoCommit}; # get/read
"AutoCommit" (boolean)
If true, then database changes cannot be rolled-back (undone). If
false, then database changes automatically occur within a "transac-
tion", which must either be committed or rolled back using the
"commit" or "rollback" methods.
Drivers should always default to "AutoCommit" mode (an unfortunate
choice largely forced on the DBI by ODBC and JDBC conventions.)
Attempting to set "AutoCommit" to an unsupported value is a fatal
error. This is an important feature of the DBI. Applications that
need full transaction behavior can set "$dbh-">"{AutoCommit} = 0"
(or set "AutoCommit" to 0 via "connect") without having to check
that the value was assigned successfully.
For the purposes of this description, we can divide databases into
three categories:
Databases which don't support transactions at all.
Databases in which a transaction is always active.
Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).
* Databases which don't support transactions at all
For these databases, attempting to turn "AutoCommit" off is a fatal
error. "commit" and "rollback" both issue warnings about being
ineffective while "AutoCommit" is in effect.
* Databases in which a transaction is always active
These are typically mainstream commercial relational databases with
"ANSI standard" transaction behavior. If "AutoCommit" is off, then
changes to the database won't have any lasting effect unless "com-
mit" is called (but see also "disconnect"). If "rollback" is called
then any changes since the last commit are undone.
If "AutoCommit" is on, then the effect is the same as if the DBI
called "commit" automatically after every successful database oper-
ation. So calling "commit" or "rollback" explicitly while "AutoCom-
mit" is on would be ineffective because the changes would have
already been commited.
Changing "AutoCommit" from off to on will trigger a "commit".
For databases which don't support a specific auto-commit mode, the
driver has to commit each statement automatically using an explicit
"COMMIT" after it completes successfully (and roll it back using an
explicit "ROLLBACK" if it fails). The error information reported
to the application will correspond to the statement which was exe-
cuted, unless it succeeded and the commit or rollback failed.
* Databases in which a transaction must be explicitly started
For these databases, the intention is to have them act like
databases in which a transaction is always active (as described
above).
To do this, the driver will automatically begin an explicit trans-
action when "AutoCommit" is turned off, or after a "commit" or
"rollback" (or when the application issues the next database opera-
tion after one of those events).
In this way, the application does not have to treat these databases
as a special case.
See "commit", "disconnect" and "Transactions" for other important
notes about transactions.
"Driver" (handle)
Holds the handle of the parent driver. The only recommended use for
this is to find the name of the driver using:
$dbh->{Driver}->{Name}
"Name" (string)
Holds the "name" of the database. Usually (and recommended to be)
the same as the ""dbi:DriverName:..."" string used to connect to
the database, but with the leading ""dbi:DriverName:"" removed.
"Statement" (string, read-only)
Returns the statement string passed to the most recent "prepare"
method called in this database handle, even if that method failed.
This is especially useful where "RaiseError" is enabled and the
exception handler checks $@ and sees that a 'prepare' method call
failed.
"RowCacheSize" (integer)
A hint to the driver indicating the size of the local row cache
that the application would like the driver to use for future
"SELECT" statements. If a row cache is not implemented, then set-
ting "RowCacheSize" is ignored and getting the value returns
"undef".
Some "RowCacheSize" values have special meaning, as follows:
0 - Automatically determine a reasonable cache size for each C<SELECT>
1 - Disable the local row cache
>1 - Cache this many rows
<0 - Cache as many rows that will fit into this much memory for each C<SELECT>.
Note that large cache sizes may require a very large amount of mem-
ory (cached rows * maximum size of row). Also, a large cache will
cause a longer delay not only for the first fetch, but also when-
ever the cache needs refilling.
See also the "RowsInCache" statement handle attribute.
DBI STATEMENT HANDLE OBJECTS
This section lists the methods and attributes associated with DBI
statement handles.
Statement Handle Methods
The DBI defines the following methods for use on DBI statement handles:
"bind_param"
$rc = $sth->bind_param($p_num, $bind_value) or die $sth->errstr;
$rv = $sth->bind_param($p_num, $bind_value, \%attr) or ...
$rv = $sth->bind_param($p_num, $bind_value, $bind_type) or ...
The "bind_param" method can be used to bind a value with a place-
holder embedded in the prepared statement. Placeholders are indi-
cated with question mark character ("?"). For example:
$dbh->{RaiseError} = 1; # save having to check each method call
$sth = $dbh->prepare("SELECT name, age FROM people WHERE name LIKE ?");
$sth->bind_param(1, "John%"); # placeholders are numbered from 1
$sth->execute;
DBI::dump_results($sth);
Note that the "?" is not enclosed in quotation marks, even when the
placeholder represents a string. Some drivers also allow place-
holders like ":"name and ":"n (e.g., ":1", ":2", and so on) in
addition to "?", but their use is not portable. Undefined bind
values or "undef" can be used to indicate null values.
Some drivers do not support placeholders.
With most drivers, placeholders can't be used for any element of a
statement that would prevent the database server from validating
the statement and creating a query execution plan for it. For
example:
"SELECT name, age FROM ?" # wrong (will probably fail)
"SELECT name, ? FROM people" # wrong (but may not 'fail')
Also, placeholders can only represent single scalar values. For
example, the following statement won't work as expected for more
than one value:
"SELECT name, age FROM people WHERE name IN (?)" # wrong
Data Types for Placeholders
The "\%attr" parameter can be used to hint at the data type the
placeholder should have. Typically, the driver is only interested
in knowing if the placeholder should be bound as a number or a
string.
$sth->bind_param(1, $value, { TYPE => SQL_INTEGER });
As a short-cut for this common case, the data type can be passed
directly, in place of the "\%attr" hash reference. This example is
equivalent to the one above:
$sth->bind_param(1, $value, SQL_INTEGER);
The "TYPE" value indicates the standard (non-driver-specific) type
for this parameter. To specify the driver-specific type, the driver
may support a driver-specific attribute, such as "{ ora_type =">"
97 }". The data type for a placeholder cannot be changed after the
first "bind_param" call. However, it can be left unspecified, in
which case it defaults to the previous value.
The SQL_INTEGER and other related constants can be imported using
use DBI qw(:sql_types);
See "DBI Constants" for more information.
Perl only has string and number scalar data types. All database
types that aren't numbers are bound as strings and must be in a
format the database will understand.
As an alternative to specifying the data type in the "bind_param"
call, you can let the driver pass the value as the default type
("VARCHAR"). You can then use an SQL function to convert the type
within the statement. For example:
INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))
The "CONVERT" function used here is just an example. The actual
function and syntax will vary between different databases and is
non-portable.
See also "Placeholders and Bind Values" for more information.
"bind_param_inout"
$rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len) or die $sth->errstr;
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr) or ...
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) or ...
This method acts like "bind_param", but also enables values to be
updated by the statement. The statement is typically a call to a
stored procedure. The $bind_value must be passed as a reference to
the actual value to be used.
Note that unlike "bind_param", the $bind_value variable is not read
when "bind_param_inout" is called. Instead, the value in the
variable is read at the time "execute" is called.
The additional $max_len parameter specifies the minimum amount of
memory to allocate to $bind_value for the new value. If the value
returned from the database is too big to fit, then the execution
should fail. If unsure what value to use, pick a generous length,
i.e., a length larger than the longest value that would ever be
returned. The only cost of using a larger value than needed is
wasted memory.
It is expected that few drivers will support this method. The only
driver currently known to do so is DBD::Oracle (DBD::ODBC may sup-
port it in a future release). Therefore it should not be used for
database independent applications.
Undefined values or "undef" are used to indicate null values. See
also "Placeholders and Bind Values" for more information.
"bind_param_array"
$rc = $sth->bind_param_array($p_num, $array_ref_or_value)
$rc = $sth->bind_param_array($p_num, $array_ref_or_value, \%attr)
$rc = $sth->bind_param_array($p_num, $array_ref_or_value, $bind_type)
The "bind_param_array" method is used to bind an array of values to
a placeholder embedded in the prepared statement which is to be
executed with "execute_array". For example:
$dbh->{RaiseError} = 1; # save having to check each method call
$sth = $dbh->prepare("INSERT INTO staff (first_name, last_name, dept) VALUES(?, ?, ?)");
$sth->bind_param_array(1, [ 'John', 'Mary', 'Tim' ]);
$sth->bind_param_array(2, [ 'Booth', 'Todd', 'Robinson' ]);
$sth->bind_param_array(3, "SALES"); # scalar will be reused for each row
my @tuple_status;
$sth->execute_array( { ArrayTupleStatus => \@tuple_status } );
The %attr argument is the same as defined for "bind_param". Refer
to "bind_param" for general details on using placeholders.
Each array bound to the statement must have the same number of ele-
ments. Some drivers may define a method attribute to relax this
safety check.
Scalar values, including "undef", may also be bound by
"bind_param_array". In which case the same value will be used for
each "execute" call. Driver-specific implementations may behave
differently, e.g., when binding to a stored procedure call, some
databases permit mixing scalars and arrays as arguments.
The default implementation provided by DBI (for drivers that have
not implemented array binding) is to iteratively "execute" for each
parameter tuple provided in the bound arrays. Drivers may provide
more optimized implementations using whatever bulk operation sup-
port the database API provides. The default driver behaviour should
match the default DBI behaviour, but always consult your driver
documentation as there may be driver specific issues to consider.
Note that the default implementation currently only supports non-
data returning statements. Also, "bind_param_array" and
"bind_param" cannot be mixed in the same statement execution, and
"bind_param_array" must be used with "execute_array"; using
"bind_param_array" will have no effect for "execute".
"execute"
$rv = $sth->execute or die $sth->errstr;
$rv = $sth->execute(@bind_values) or die $sth->errstr;
Perform whatever processing is necessary to execute the prepared
statement. An "undef" is returned if an error occurs. A
successful "execute" always returns true regardless of the number
of rows affected, even if it's zero (see below). It is always
important to check the return status of "execute" (and most other
DBI methods) for errors.
For a non-"SELECT" statement, "execute" returns the number of rows
affected, if known. If no rows were affected, then "execute"
returns "0E0", which Perl will treat as 0 but will regard as true.
Note that it is not an error for no rows to be affected by a state-
ment. If the number of rows affected is not known, then "execute"
returns -1.
For "SELECT" statements, execute simply "starts" the query within
the database engine. Use one of the fetch methods to retreive the
data after calling "execute". The "execute" method does not return
the number of rows that will be returned by the query (because most
databases can't tell in advance), it simply returns a true value.
If any arguments are given, then "execute" will effectively call
"bind_param" for each value before executing the statement. Values
bound in this way are usually treated as "SQL_VARCHAR" types unless
the driver can determine the correct type (which is rare), or
unless "bind_param" (or "bind_param_inout") has already been used
to specify the type.
"execute_array"
$rv = $sth->execute_array(\%attr) or die $sth->errstr;
$rv = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;
Execute the prepared statement for each parameter tuple provided
either in the @bind_values, or by prior calls to
"bind_param_array".
An "undef" is returned if an error occurs. A successful "exe-
cute_array" always returns true regardless of the number of rows
affected, even if it's zero (see below). It is always important to
check the return status of "execute_array" (and most other DBI
methods) for errors.
Parameters may be supplied either by prior calls to
"bind_param_array", or in the @bind_values argument. The values
supplied may be either scalars, or arrayrefs. See
"bind_param_array" for details.
The supplied "\%attr" hashref currently supports only the "ArrayTu-
pleStatus" attribute, which should specify an arrayref to receive
the status of each parameter tuple bound to the statement. For
parameter tuples which are successfully executed, the element at
the same ordinal position in the status array will return the
resulting rowcount.
For example:
$sth = $dbh->prepare("INSERT INTO staff (first_name, last_name) VALUES (?, ?)");
my @tuple_status;
$sth->execute_array(
{ ArrayTupleStatus => \@tuple_status },
\@first_names,
\@last_names,
);
If a parameter tuple causes an error, the associated status array
element will be set to an arrayref of [ $sth->err, $sth->errstr ]
returned by the failed execution. If any tuple returns an error,
"execute_array" will return "undef" after it has executed all the
parameter tuples. In that case, the application should inspect the
status array to determine which parameter tuples failed.
If no "ArrayTupleStatus" is provided, "execute_array" will return
"undef" on the first occurance of a parameter tuple causing an
error. [XXX This may change as it doesn't match the behaviour of
drivers which use bulk operation API to ship the data to the
server.]
If all parameter tuples are successfully executed, "execute_array"
returns the sum of the number of rows affected by all the parameter
tuples, if known. If no rows were affected, then "execute" returns
"0E0", which Perl will treat as 0 but will regard as true. Note
that it is not an error for no rows to be affected by a statement.
If the number of rows affected is not known, then "execute_array"
may return a negative number. Applications should not rely on the
returned value to indicate actual total rowcounts, but use the
"ArrayTupleStatus" and explicitly inspect each returned element of
the status array.
Support for data returning statements is driver-specific and sub-
ject to change. At present, the default implementation provided by
DBI only supports non-data returning statements.
If any @bind_values are given, then "execute_array" will effec-
tively call "bind_param_array" for each value before executing the
statement. Values bound in this way are usually treated as
"SQL_VARCHAR" types unless the driver can determine the correct
type (which is rare), or unless "bind_param", "bind_param_inout",
"bind_param_array", or "bind_param_inout_array" has already been
used to specify the type.
Transaction semantics using array binding are driver and database
specific. If "AutoCommit" is on, the default DBI implementation
will cause each parameter tuple to be inidividually committed (or
rolled back in the event of an error). If "AutoCommit" is off, the
application is responsible for explicitly committing the entire set
of bound parameter tuples. Note that different drivers and
databases may have different behaviors when some parameter tuples
cause failures. In some cases, the driver or database may automati-
cally rollback the effect of all prior parameter tuples that suc-
ceeded in the transaction; other drivers or databases may retain
the effect of prior successfully executed parameter tuples. Be sure
to check your driver and database for its specific behavior.
Note that, in general, performance will usually be better with
"AutoCommit" turned off, and using explicit "commit" after each
"execute_array" call.
"fetchrow_arrayref"
$ary_ref = $sth->fetchrow_arrayref;
$ary_ref = $sth->fetch; # alias
Fetches the next row of data and returns a reference to an array
holding the field values. Null fields are returned as "undef" val-
ues in the array. This is the fastest way to fetch data, particu-
larly if used with "$sth-">"bind_columns".
If there are no more rows or if an error occurs, then
"fetchrow_arrayref" returns an "undef". You should check
"$sth-">"err" afterwards (or use the "RaiseError" attribute) to
discover if the "undef" returned was due to an error.
Note that the same array reference is returned for each fetch, so
don't store the reference and then use it after a later fetch.
Also, the elements of the array are also reused for each row, so
take care if you want to take a reference to an element. See also
"bind_columns".
"fetchrow_array"
@ary = $sth->fetchrow_array;
An alternative to "fetchrow_arrayref". Fetches the next row of data
and returns it as a list containing the field values. Null fields
are returned as "undef" values in the list.
If there are no more rows or if an error occurs, then
"fetchrow_array" returns an empty list. You should check
"$sth-">"err" afterwards (or use the "RaiseError" attribute) to
discover if the empty list returned was due to an error.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return the
value of the first column or the last. So don't do that. Also, in
a scalar context, an "undef" is returned if there are no more rows
or if an error occurred. That "undef" can't be distinguished from
an "undef" returned because the first field value was NULL. For
these reasons you should exercise some caution if you use
"fetchrow_array" in a scalar context.
"fetchrow_hashref"
$hash_ref = $sth->fetchrow_hashref;
$hash_ref = $sth->fetchrow_hashref($name);
An alternative to "fetchrow_arrayref". Fetches the next row of data
and returns it as a reference to a hash containing field name and
field value pairs. Null fields are returned as "undef" values in
the hash.
If there are no more rows or if an error occurs, then
"fetchrow_hashref" returns an "undef". You should check
"$sth-">"err" afterwards (or use the "RaiseError" attribute) to
discover if the "undef" returned was due to an error.
The optional $name parameter specifies the name of the statement
handle attribute. For historical reasons it defaults to ""NAME"",
however using either ""NAME_lc"" or ""NAME_uc"" is recomended for
portability.
The keys of the hash are the same names returned by
"$sth-">"{$name}". If more than one field has the same name, there
will only be one entry in the returned hash for those fields.
Because of the extra work "fetchrow_hashref" and Perl have to per-
form, it is not as efficient as "fetchrow_arrayref" or
"fetchrow_array".
Currently, a new hash reference is returned for each row. This
will change in the future to return the same hash ref each time, so
don't rely on the current behaviour.
"fetchall_arrayref"
$tbl_ary_ref = $sth->fetchall_arrayref;
$tbl_ary_ref = $sth->fetchall_arrayref( $slice );
$tbl_ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
The "fetchall_arrayref" method can be used to fetch all the data to
be returned from a prepared and executed statement handle. It
returns a reference to an array that contains one reference per
row.
If there are no rows to return, "fetchall_arrayref" returns a ref-
erence to an empty array. If an error occurs, "fetchall_arrayref"
returns the data fetched thus far, which may be none. You should
check "$sth-">"err" afterwards (or use the "RaiseError" attribute)
to discover if the data is complete or was truncated due to an
error.
If $slice is an array reference, "fetchall_arrayref" uses
"fetchrow_arrayref" to fetch each row as an array ref. If the
parameter array is not empty then it is used as a slice to select
individual columns by perl array index number (starting at 0,
unlike column and parameter numbers which start at 1).
With no parameters, or if $slice is undefined, "fetchall_arrayref"
acts as if passed an empty array ref.
If $slice is a hash reference, "fetchall_arrayref" uses
"fetchrow_hashref" to fetch each row as a hash reference. If the
$slice hash is empty then fetchrow_hashref() is simply called in a
tight loop and the keys in the hashes have whatever name lettercase
is returned by default from fetchrow_hashref. (See "FetchHashKey-
Name" attribute.) If the $slice hash is not empty, then it is used
as a slice to select individual columns by name. The values of the
hash should be set to 1. The key names of the returned hashes
match the letter case of the names in the parameter hash, regard-
less of the "FetchHashKeyName" attribute.
For example, to fetch just the first column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([0]);
To fetch the second to last and last column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([-2,-1]);
To fetch all fields of every row as a hash ref:
$tbl_ary_ref = $sth->fetchall_arrayref({});
To fetch only the fields called "foo" and "bar" of every row as a
hash ref (with keys named "foo" and "BAR"):
$tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, BAR=>1 });
The first two examples return a reference to an array of array
refs. The third and forth return a reference to an array of hash
refs.
If $max_rows is defined and greater than or equal to zero then it
is used to limit the number of rows fetched before returning.
fetchall_arrayref() can then be called again to fetch more rows.
This is especially useful when you need the better performance of
fetchall_arrayref() but don't have enough memory to fetch and
return all the rows in one go. Here's an example:
my $rows = []; # cache for batches of rows
while( my $row = ( shift(@$rows) || # get row from cache, or reload cache:
shift(@{$rows=$sth->fetchall_arrayref(undef,10_000)||[]) )
) {
...
}
That is the fastest way to fetch and process lots of rows using the
DBI.
"fetchall_hashref"
$hash_ref = $sth->fetchall_hashref($key_field);
The "fetchall_hashref" method can be used to fetch all the data to
be returned from a prepared and executed statement handle. It
returns a reference to a hash that contains, at most, one entry per
row.
If there are no rows to return, "fetchall_hashref" returns a refer-
ence to an empty hash. If an error occurs, "fetchall_hashref"
returns the data fetched thus far, which may be none. You should
check "$sth-">"err" afterwards (or use the "RaiseError" attribute)
to discover if the data is complete or was truncated due to an
error.
The $key_field parameter provides the name of the field that holds
the value to be used for the key for the returned hash. For exam-
ple:
$dbh->{FetchHashKeyName} = 'NAME_lc';
$sth = $dbh->prepare("SELECT FOO, BAR, ID, NAME, BAZ FROM TABLE");
$hash_ref = $sth->fetchall_hashref('id');
print "Name for id 42 is $hash_ref->{42}->{name}\n";
The $key_field parameter can also be specified as an integer column
number (counting from 1). If $key_field doesn't match any column
in the statement, as a name first then as a number, then an error
is returned.
This method is normally used only where the key field value for
each row is unique. If multiple rows are returned with the same
value for the key field then later rows overwrite earlier ones.
"finish"
$rc = $sth->finish;
Indicates that no more data will be fetched from this statement
handle before it is either executed again or destroyed. The "fin-
ish" method is rarely needed, but can sometimes be helpful in very
specific situations to allow the server to free up resources (such
as sort buffers).
When all the data has been fetched from a "SELECT" statement, the
driver should automatically call "finish" for you. So you should
not normally need to call it explicitly except when you know that
you've not fetched all the data from a statement handle. The most
common example is when you only want to fetch one row, but in that
case the "selectrow_*" methods may be better anyway. Adding calls
to "finish" after each fetch loop is a common mistake, don't do it,
it can mask genuine problems like uncaught fetch errors.
Consider a query like:
SELECT foo FROM table WHERE bar=? ORDER BY foo
where you want to select just the first (smallest) "foo" value from
a very large table. When executed, the database server will have to
use temporary buffer space to store the sorted rows. If, after exe-
cuting the handle and selecting one row, the handle won't be re-
executed for some time and won't be destroyed, the "finish" method
can be used to tell the server that the buffer space can be freed.
Calling "finish" resets the "Active" attribute for the statement.
It may also make some statement handle attributes (such as "NAME"
and "TYPE") unavailable if they have not already been accessed (and
thus cached).
The "finish" method does not affect the transaction status of the
database connection. It has nothing to do with transactions. It's
mostly an internal "housekeeping" method that is rarely needed.
See also "disconnect" and the "Active" attribute.
The "finish" method should have been called "cancel_select".
"rows"
$rv = $sth->rows;
Returns the number of rows affected by the last row affecting com-
mand, or -1 if the number of rows is not known or not available.
Generally, you can only rely on a row count after a non-"SELECT"
"execute" (for some specific operations like "UPDATE" and
"DELETE"), or after fetching all the rows of a "SELECT" statement.
For "SELECT" statements, it is generally not possible to know how
many rows will be returned except by fetching them all. Some
drivers will return the number of rows the application has fetched
so far, but others may return -1 until all rows have been fetched.
So use of the "rows" method or $DBI::rows with "SELECT" statements
is not recommended.
One alternative method to get a row count for a "SELECT" is to exe-
cute a "SELECT COUNT(*) FROM ..." SQL statement with the same "..."
as your query and then fetch the row count from that.
"bind_col"
$rc = $sth->bind_col($column_number, \$var_to_bind);
Binds an output column (field) of a "SELECT" statement to a Perl
variable. See "bind_columns" below for an example. Note that col-
umn numbers count up from 1.
Whenever a row is fetched from the database, the corresponding Perl
variable is automatically updated. There is no need to fetch and
assign the values manually. The binding is performed at a very low
level using Perl aliasing so there is no extra copying taking
place. This makes using bound variables very efficient.
For maximum portability between drivers, "bind_col" should be
called after "execute". This restriction may be removed in a later
version of the DBI.
You do not need to bind output columns in order to fetch data, but
it can be useful for some applications which need either maximum
performance or greater clarity of code. The "bind_param" method
performs a similar but opposite function for input variables.
"bind_columns"
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
Calls "bind_col" for each column of the "SELECT" statement. The
"bind_columns" method will die if the number of references does not
match the number of fields.
For maximum portability between drivers, "bind_columns" should be
called after "execute".
For example:
$dbh->{RaiseError} = 1; # do this, or check every call for errors
$sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
$sth->execute;
my ($region, $sales);
# Bind Perl variables to columns:
$rv = $sth->bind_columns(\$region, \$sales);
# you can also use Perl's \(...) syntax (see perlref docs):
# $sth->bind_columns(\($region, $sales));
# Column binding is the most efficient way to fetch data
while ($sth->fetch) {
print "$region: $sales\n";
}
For compatibility with old scripts, the first parameter will be
ignored if it is "undef" or a hash reference.
Here's a more fancy example that binds columns to the values inside
a hash (thanks to H.Merijn Brand):
$sth->execute;
my %row;
$sth->bind_columns( \( @row{ @{$sth->{NAME_lc} } } ));
while ($sth->fetch) {
print "$row{region}: $row{sales}\n";
}
"dump_results"
$rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);
Fetches all the rows from $sth, calls "DBI::neat_list" for each
row, and prints the results to $fh (defaults to "STDOUT") separated
by $lsep (default "\n"). $fsep defaults to ", " and $maxlen
defaults to 35.
This method is designed as a handy utility for prototyping and
testing queries. Since it uses "neat_list" to format and edit the
string for reading by humans, it is not recomended for data trans-
fer applications.
Statement Handle Attributes
This section describes attributes specific to statement handles. Most
of these attributes are read-only.
Changes to these statement handle attributes do not affect any other
existing or future statement handles.
Attempting to set or get the value of an unknown attribute is fatal,
except for private driver specific attributes (which all have names
starting with a lowercase letter).
Example:
... = $h->{NUM_OF_FIELDS}; # get/read
Note that some drivers cannot provide valid values for some or all of
these attributes until after "$sth-">"execute" has been called.
See also "finish" to learn more about the effect it may have on some
attributes.
"NUM_OF_FIELDS" (integer, read-only)
Number of fields (columns) in the data the prepared statement may
return. Statements that don't return rows of data, like "DELETE"
and "CREATE" set "NUM_OF_FIELDS" to 0.
"NUM_OF_PARAMS" (integer, read-only)
The number of parameters (placeholders) in the prepared statement.
See SUBSTITUTION VARIABLES below for more details.
"NAME" (array-ref, read-only)
Returns a reference to an array of field names for each column. The
names may contain spaces but should not be truncated or have any
trailing space. Note that the names have the letter case (upper,
lower or mixed) as returned by the driver being used. Portable
applications should use "NAME_lc" or "NAME_uc".
print "First column name: $sth->{NAME}->[0]\n";
"NAME_lc" (array-ref, read-only)
Like "NAME" but always returns lowercase names.
"NAME_uc" (array-ref, read-only)
Like "NAME" but always returns uppercase names.
"NAME_hash" (hash-ref, read-only)
"NAME_lc_hash" (hash-ref, read-only)
"NAME_uc_hash" (hash-ref, read-only)
The "NAME_hash", "NAME_lc_hash", and "NAME_uc_hash" attributes
return column name information as a reference to a hash.
The keys of the hash are the names of the columns. The letter case
of the keys corresponds to the letter case returned by the "NAME",
"NAME_lc", and "NAME_uc" attributes respectively (as described
above).
The value of each hash entry is the perl index number of the corre-
sponding column (counting from 0). For example:
$sth = $dbh->prepare("select Id, Name from table");
$sth->execute;
@row = $sth->fetchrow_array;
print "Name $row[ $sth->{NAME_lc_hash}{name} ]\n";
"TYPE" (array-ref, read-only)
Returns a reference to an array of integer values for each column.
The value indicates the data type of the corresponding column.
The values correspond to the international standards (ANSI X3.135
and ISO/IEC 9075) which, in general terms, means ODBC. Driver-spe-
cific types that don't exactly match standard types should gener-
ally return the same values as an ODBC driver supplied by the mak-
ers of the database. That might include private type numbers in
ranges the vendor has officially registered with the ISO working
group:
ftp://sqlstandards.org/SC32/SQL_Registry/
Where there's no vendor-supplied ODBC driver to be compatible with,
the DBI driver can use type numbers in the range that is now offi-
cially reserved for use by the DBI: -9999 to -9000.
All possible values for "TYPE" should have at least one entry in
the output of the "type_info_all" method (see "type_info_all").
"PRECISION" (array-ref, read-only)
Returns a reference to an array of integer values for each column.
For non-numeric columns, the value generally refers to either the
maximum length or the defined length of the column. For numeric
columns, the value refers to the maximum number of significant dig-
its used by the data type (without considering a sign character or
decimal point). Note that for floating point types (REAL, FLOAT,
DOUBLE), the "display size" can be up to 7 characters greater than
the precision. (for the sign + decimal point + the letter E + a
sign + 2 or 3 digits).
"SCALE" (array-ref, read-only)
Returns a reference to an array of integer values for each column.
NULL ("undef") values indicate columns where scale is not applica-
ble.
"NULLABLE" (array-ref, read-only)
Returns a reference to an array indicating the possibility of each
column returning a null. Possible values are 0 (or an empty
string) = no, 1 = yes, 2 = unknown.
print "First column may return NULL\n" if $sth->{NULLABLE}->[0];
"CursorName" (string, read-only)
Returns the name of the cursor associated with the statement han-
dle, if available. If not available or if the database driver does
not support the "where current of ..." SQL syntax, then it returns
"undef".
"Database" (dbh, read-only)
Returns the parent $dbh of the statement handle.
"ParamValues" (hash ref, read-only)
Returns a reference to a hash containing the values currently bound
to placeholders. Returns undef if not supported by the driver. If
the driver does support "ParamValues" but no values have been bound
yet then either undef or an empty hash may be returned.
See "ShowErrorStatement" for an example of how this is used.
It is possible that the values in the hash returned by "ParamVal-
ues" are not exactly the same as those passed to bind_param() or
execute(). The driver may have modified the values in some way
based on the TYPE the value was bound with. For example a floating
point value bound as an SQL_INTEGER type may be returned as an
integer.
"Statement" (string, read-only)
Returns the statement string passed to the "prepare" method.
"RowsInCache" (integer, read-only)
If the driver supports a local row cache for "SELECT" statements,
then this attribute holds the number of un-fetched rows in the
cache. If the driver doesn't, then it returns "undef". Note that
some drivers pre-fetch rows on execute, whereas others wait till
the first fetch.
See also the "RowCacheSize" database handle attribute.
FURTHER INFORMATION
Catalog Methods
An application can retrieve metadata information from the DBMS by issu-
ing appropriate queries on the views of the Information Schema. Unfor-
tunately, "INFORMATION_SCHEMA" views are seldom supported by the DBMS.
Special methods (catalog methods) are available to return result sets
for a small but important portion of that metadata:
column_info
foreign_key_info
primary_key_info
table_info
All catalog methods accept arguments in order to restrict the result
sets. Passing "undef" to an optional argument does not constrain the
search for that argument. However, an empty string ('') is treated as
a regular search criteria and will only match an empty value.
Note: SQL/CLI and ODBC differ in the handling of empty strings. An
empty string will not restrict the result set in SQL/CLI.
Most arguments in the catalog methods accept only ordinary values, e.g.
the arguments of "primary_key_info()". Such arguments are treated as a
literal string, i.e. the case is significant and quote characters are
taken literally.
Some arguments in the catalog methods accept search patterns (strings
containing '_' and/or '%'), e.g. the $table argument of "col-
umn_info()". Passing '%' is equivalent to leaving the argument
"undef".
Caveat: The underscore ('_') is valid and often used in SQL identi-
fiers. Passing such a value to a search pattern argument may return
more rows than expected! To include pattern characters as literals,
they must be preceded by an escape character which can be achieved with
$esc = $dbh->get_info( 14 ); # SQL_SEARCH_PATTERN_ESCAPE
$search_pattern =~ s/([_%])/$esc$1/g;
The ODBC and SQL/CLI specifications define a way to change the default
behavior described above: All arguments (except list value arguments)
are treated as identifier if the "SQL_ATTR_METADATA_ID" attribute is
set to "SQL_TRUE". Quoted identifiers are very similar to ordinary
values, i.e. their body (the string within the quotes) is interpreted
literally. Unquoted identifiers are compared in UPPERCASE.
The DBI (currently) does not support the "SQL_ATTR_METADATA_ID"
attribute, i.e. it behaves like an ODBC driver where "SQL_ATTR_META-
DATA_ID" is set to "SQL_FALSE".
Transactions
Transactions are a fundamental part of any robust database system. They
protect against errors and database corruption by ensuring that sets of
related changes to the database take place in atomic (indivisible,
all-or-nothing) units.
This section applies to databases that support transactions and where
"AutoCommit" is off. See "AutoCommit" for details of using "AutoCom-
mit" with various types of databases.
The recommended way to implement robust transactions in Perl applica-
tions is to use "RaiseError" and "eval { ... }" (which is very fast,
unlike "eval "...""). For example:
$dbh->{AutoCommit} = 0; # enable transactions, if possible
$dbh->{RaiseError} = 1;
eval {
foo(...) # do lots of work here
bar(...) # including inserts
baz(...) # and updates
$dbh->commit; # commit the changes if we get this far
};
if ($@) {
warn "Transaction aborted because $@";
$dbh->rollback; # undo the incomplete changes
# add other application on-error-clean-up code here
}
If the "RaiseError" attribute is not set, then DBI calls would need to
be manually checked for errors, typically like this:
$h->method(@args) or die $h->errstr;
With "RaiseError" set, the DBI will automatically "die" if any DBI
method call on that handle (or a child handle) fails, so you don't have
to test the return value of each method call. See "RaiseError" for more
details.
A major advantage of the "eval" approach is that the transaction will
be properly rolled back if any code (not just DBI calls) in the inner
application dies for any reason. The major advantage of using the
"$h-">"{RaiseError}" attribute is that all DBI calls will be checked
automatically. Both techniques are strongly recommended.
After calling "commit" or "rollback" many drivers will not let you
fetch from a previously active "SELECT" statement handle that's a child
of the same database handle. A typical way round this is to connect the
the database twice and use one connection for "SELECT" statements.
See "AutoCommit" and "disconnect" for other important information about
transactions.
Handling BLOB / LONG / Memo Fields
Many databases support "blob" (binary large objects), "long", or simi-
lar datatypes for holding very long strings or large amounts of binary
data in a single field. Some databases support variable length long
values over 2,000,000,000 bytes in length.
Since values of that size can't usually be held in memory, and because
databases can't usually know in advance the length of the longest long
that will be returned from a "SELECT" statement (unlike other data
types), some special handling is required.
In this situation, the value of the "$h-">"{LongReadLen}" attribute is
used to determine how much buffer space to allocate when fetching such
fields. The "$h-">"{LongTruncOk}" attribute is used to determine how
to behave if a fetched value can't fit into the buffer.
When trying to insert long or binary values, placeholders should be
used since there are often limits on the maximum size of an "INSERT"
statement and the "quote" method generally can't cope with binary data.
See "Placeholders and Bind Values".
Simple Examples
Here's a complete example program to select and fetch some data:
my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password)
or die "Can't connect to $data_source: $DBI::errstr";
my $sth = $dbh->prepare( q{
SELECT name, phone
FROM mytelbook
}) or die "Can't prepare statement: $DBI::errstr";
my $rc = $sth->execute
or die "Can't execute statement: $DBI::errstr";
print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
print "Field names: @{ $sth->{NAME} }\n";
while (($name, $phone) = $sth->fetchrow_array) {
print "$name: $phone\n";
}
# check for problems which may have terminated the fetch early
die $sth->errstr if $sth->err;
$dbh->disconnect;
Here's a complete example program to insert some data from a file.
(This example uses "RaiseError" to avoid needing to check each call).
my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
RaiseError => 1, AutoCommit => 0
});
my $sth = $dbh->prepare( q{
INSERT INTO table (name, phone) VALUES (?, ?)
});
open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
while (<FH>) {
chomp;
my ($name, $phone) = split /,/;
$sth->execute($name, $phone);
}
close FH;
$dbh->commit;
$dbh->disconnect;
Here's how to convert fetched NULLs (undefined values) into empty
strings:
while($row = $sth->fetchrow_arrayref) {
# this is a fast and simple way to deal with nulls:
foreach (@$row) { $_ = '' unless defined }
print "@$row\n";
}
The "q{...}" style quoting used in these examples avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote like
"qq{...}" operator if you want to interpolate variables into the
string. See "Quote and Quote-like Operators" in perlop for more
details.
Threads and Thread Safety
Perl 5.7 and later support a new threading model called iThreads. (The
old and fatally flawed "5.005 style" threads are not supported by the
DBI.)
In the iThreads model each thread has it's own copy of the perl inter-
preter. When a new thread is created the original perl interpreter is
'cloned' to create a new copy for the new thread.
If the DBI and drivers are loaded and handles created before the thread
is created then it will get a cloned copy of the DBI, the drivers and
the handles.
However, the internal pointer data within the handles will refer to the
DBI and drivers in the original interpreter. Using those handles in the
new interpreter thread is not safe, so the DBI detects this and croaks
on any method call using handles that don't belong to the current
thread (except for DESTROY).
Because of this (possibly temporary) restriction, newly created threads
must make their own connctions to the database. Handles can't be shared
across threads.
But BEWARE, some underlying database APIs (the code the DBD driver uses
to talk to the database, often supplied by the database vendor) are not
thread safe. If it's not thread safe, then allowing more than one
thread to enter the code at the same time may cause subtle/serious
problems. In some cases allowing more than one thread to enter the
code, even if not at the same time, can cause problems. You have been
warned.
Using DBI with perl threads is not yet recommended for production envi-
ronments.
Signal Handling and Canceling Operations
The first thing to say is that signal handling in Perl is currently not
safe. There is always a small risk of Perl crashing and/or core dumping
when, or after, handling a signal. (The risk was reduced with 5.004_04
but is still present.)
The two most common uses of signals in relation to the DBI are for can-
celing operations when the user types Ctrl-C (interrupt), and for
implementing a timeout using "alarm()" and $SIG{ALRM}.
To assist in implementing these operations, the DBI provides a "cancel"
method for statement handles. The "cancel" method should abort the cur-
rent operation and is designed to be called from a signal handler.
However, it must be stressed that: a) few drivers implement this at the
moment (the DBI provides a default method that just returns "undef");
and b) even if implemented, there is still a possibility that the
statement handle, and possibly the parent database handle, will not be
usable afterwards.
If "cancel" returns true, then it has successfully invoked the database
engine's own cancel function. If it returns false, then "cancel"
failed. If it returns "undef", then the database engine does not have
cancel implemented.
Subclassing the DBI
DBI can be subclassed and extended just like any other object oriented
module. Before we talk about how to do that, it's important to be
clear about how the DBI classes and how they work together.
By default "$dbh = DBI-">"connect(...)" returns a $dbh blessed into the
"DBI::db" class. And the "$dbh-">"prepare" method returns an $sth
blessed into the "DBI::st" class (actually it simply changes the last
four characters of the calling handle class to be "::st").
The leading '"DBI"' is known as the 'root class' and the extra '"::db"'
or '"::st"' are the 'handle type suffixes'. If you want to subclass the
DBI you'll need to put your overriding methods into the appropriate
classes. For example, if you want to use a root class of "MySubDBI"
and override the do(), prepare() and execute() methods, then your do()
and prepare() methods should be in the "MySubDBI::db" class and the
execute() method should be in the "MySubDBI::st" class.
To setup the inheritance hierarchy the @ISA variable in "MySubDBI::db"
should include "DBI::db" and the @ISA variable in "MySubDBI::st" should
include "DBI::st". The "MySubDBI" root class itself isn't currently
used for anything visible and so, apart from setting @ISA to include
"DBI", it should be left empty.
So, having put your overriding methods into the right classes, and
setup the inheritance hierarchy, how do you get the DBI to use them?
You have two choices, either a static method call using the name of
your subclass:
$dbh = MySubDBI->connect(...);
or specifying a "RootClass" attribute:
$dbh = DBI->connect(..., { RootClass => 'MySubDBI' });
The only difference between the two is that using an explicit RootClass
attribute will make the DBI automatically attempt to load a module by
that name if the class doesn't exist.
If both forms are used then the attribute takes precedence.
The when subclassing is being used then, after a successful new con-
nect, the DBI->connect method automatically calls:
$dbh->connected($dsn, $user, $pass, \%attr);
The default method does nothing. The call is made just to simplify any
post-connection setup that your subclass may want to perform.
Here's a brief example of a DBI subclass. A more thorough example can
be found in t/subclass.t in the DBI distribution.
package MySubDBI;
use strict;
use DBI;
use vars qw(@ISA);
@ISA = qw(DBI);
package MySubDBI::db;
use vars qw(@ISA);
@ISA = qw(DBI::db);
sub prepare {
my ($dbh, @args) = @_;
my $sth = $dbh->SUPER::prepare(@args)
or return;
$sth->{private_mysubdbi_info} = { foo => 'bar' };
return $sth;
}
package MySubDBI::st;
use vars qw(@ISA);
@ISA = qw(DBI::st);
sub fetch {
my ($sth, @args) = @_;
my $row = $sth->SUPER::fetch(@args)
or return;
do_something_magical_with_row_data($row)
or return $sth->set_err(1234, "The magic failed", undef, "fetch");
return $row;
}
When calling a SUPER::method that returns a handle, be careful to check
the return value before trying to do other things with it in your over-
ridden method. This is especially important if you want to set a hash
attribute on the handle, as Perl's autovivification will bite you by
(in)conveniently creating an unblessed hashref, which your method will
then return with usually baffling results later on. It's best to check
right after the call and return undef immediately on error, just like
DBI would and just like the example above.
If your method needs to record an error it should call the set_err()
method with the error code and error string, as shown in the example
above. The error code and error string will be recorded in the handle
and available via "$h-">"err" and $DBI::errstr etc. The set_err()
method always returns an undef or empty list as approriate. Since your
method should nearly always return an undef or empty list as soon as an
error is detected it's handy to simply return what set_err() returns,
as shown in the example above.
If the handle has "RaiseError", "PrintError", or "HandleError" etc. set
then the set_err() method will honour them. This means that if
"RaiseError" is set then set_err() won't return in the normal way but
will 'throw an exception' that can be caught with an "eval" block.
You can stash private data into DBI handles via "$h-">"{pri-
vate_..._*}". See the entry under "ATTRIBUTES COMMON TO ALL HANDLES"
for info and important caveats.
DEBUGGING
In addition to the "trace" method, you can enable the same trace infor-
mation by setting the "DBI_TRACE" environment variable before starting
Perl.
On Unix-like systems using a Bourne-like shell, you can do this easily
on the command line:
DBI_TRACE=2 perl your_test_script.pl
If "DBI_TRACE" is set to a non-numeric value, then it is assumed to be
a file name and the trace level will be set to 2 with all trace output
appended to that file. If the name begins with a number followed by an
equal sign ("="), then the number and the equal sign are stripped off
from the name, and the number is used to set the trace level. For exam-
ple:
DBI_TRACE=1=dbitrace.log perl your_test_script.pl
See also the "trace" method.
It can sometimes be handy to compare trace files from two different
runs of the same script. However using a tool like "diff" doesn't work
well because the trace file is full of object addresses that may differ
each run. Here's a handy little command to strip those out:
perl -pe 's/\b0x[\da-f]{6,}/0xNNNN/gi; s/\b[\da-f]{6,}/<long number>/gi'
WARNING AND ERROR MESSAGES
Fatal Errors
Can't call method "prepare" without a package or object reference
The $dbh handle you're using to call "prepare" is probably unde-
fined because the preceding "connect" failed. You should always
check the return status of DBI methods, or use the "RaiseError"
attribute.
Can't call method "execute" without a package or object reference
The $sth handle you're using to call "execute" is probably unde-
fined because the preceeding "prepare" failed. You should always
check the return status of DBI methods, or use the "RaiseError"
attribute.
DBI/DBD internal version mismatch
The DBD driver module was built with a different version of DBI
than the one currently being used. You should rebuild the DBD mod-
ule under the current version of DBI.
(Some rare platforms require "static linking". On those platforms,
there may be an old DBI or DBD driver version actually embedded in
the Perl executable being used.)
DBD driver has not implemented the AutoCommit attribute
The DBD driver implementation is incomplete. Consult the author.
Can't [sg]et %s->{%s}: unrecognised attribute
You attempted to set or get an unknown attribute of a handle. Make
sure you have spelled the attribute name correctly; case is signif-
icant (e.g., "Autocommit" is not the same as "AutoCommit").
Pure-Perl DBI
A pure-perl emulation of the DBI is included in the distribution for
people using pure-perl drivers who, for whatever reason, can't install
the compiled DBI. See DBI::PurePerl.
SEE ALSO
Driver and Database Documentation
Refer to the documentation for the DBD driver that you are using.
Refer to the SQL Language Reference Manual for the database engine that
you are using.
Standards Reference Information
More detailed information about the semantics of certain DBI methods
that are based on ODBC and SQL/CLI standards is available on-line via
microsoft.com, for ODBC, and www.jtc1sc32.org for the SQL/CLI standard:
DBI method ODBC function SQL/CLI Working Draft
---------- ------------- ---------------------
column_info SQLColumns Page 124
foreign_key_info SQLForeignKeys Page 163
get_info SQLGetInfo Page 214
primary_key_info SQLPrimaryKeys Page 254
table_info SQLTables Page 294
type_info SQLGetTypeInfo Page 239
For example, for ODBC information on SQLColumns you'd visit:
http://msdn.microsoft.com/library/en-us/odbc/htm/odbcsqlcolumns.asp
If that URL ceases to work then use the MSDN search facility at:
http://search.microsoft.com/us/dev/
and search for "SQLColumns returns" using the exact phrase option. The
link you want will probably just be called "SQLColumns" and will be
part of the Data Access SDK.
And for SQL/CLI standard information on SQLColumns you'd read page 124
of the (very large) SQL/CLI Working Draft available from:
http://www.jtc1sc32.org/sc32/jtc1sc32.nsf/Attachments/7E3B41486BD99C3488256B410064C877/$FILE/32N0744T.PDF
A hyperlinked, browsable version of the BNF syntax for SQL92 (plus
Oracle 7 SQL and PL/SQL) is available here:
http://cui.unige.ch/db-research/Enseignement/analyseinfo/SQL92/BNFindex.html
A BNF syntax for SQL3 is available here:
http://www.sqlstandards.org/SC32/WG3/Progression_Documents/Informal_working_drafts/iso-9075-2-1999.bnf
Books and Journals
Programming the Perl DBI, by Alligator Descartes and Tim Bunce.
Programming Perl 3rd Ed. by Larry Wall, Tom Christiansen & Jon Orwant.
Learning Perl by Randal Schwartz.
Dr Dobb's Journal, November 1996.
The Perl Journal, April 1997.
Perl Modules
Index of DBI related modules available from CPAN:
http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
http://search.cpan.org/search?mode=doc&query=DBI
For a good comparison of RDBMS-OO mappers and some OO-RDBMS mappers
(including Class::DBI, Alzabo, and DBIx::RecordSet in the former cate-
gory and Tangram and SPOPS in the latter) see the Perl Object-Oriented
Persistence project pages at:
http://poop.sourceforge.net
Manual Pages
perl(1), perlmod(1), perlbook(1)
Mailing List
The dbi-users mailing list is the primary means of communication among
users of the DBI and its related modules. For details send email to:
dbi-users-help@perl.org
There are typically between 700 and 900 messages per month. You have
to subscribe in order to be able to post. However you can opt for a
'post-only' subscription.
Mailing list archives (of variable quality) are held at:
http://www.xray.mpe.mpg.de/mailing-lists/dbi/
http://groups.yahoo.com/group/dbi-users
http://www.bitmechanic.com/mail-archives/dbi-users/
http://marc.theaimsgroup.com/?l=perl-dbi&r=1&w=2
http://www.mail-archive.com/dbi-users%40perl.org/
Assorted Related WWW Links
The DBI "Home Page":
http://dbi.perl.org/
Other DBI related links:
http://tegan.deltanet.com/~phlip/DBUIdoc.html
http://dc.pm.org/perl_db.html
http://wdvl.com/Authoring/DB/Intro/toc.html
http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html
http://bumppo.net/lists/macperl/1999/06/msg00197.html
Other database related links:
http://www.jcc.com/sql_stnd.html
http://cuiwww.unige.ch/OSG/info/FreeDB/FreeDB.home.html
Security, especially the "SQL Injection" attack:
http://online.securityfocus.com/infocus/1644
http://www.nextgenss.com/research/papers.html
Commercial and Data Warehouse Links
http://www.dwinfocenter.org
http://www.datawarehouse.com
http://www.datamining.org
http://www.olapcouncil.org
http://www.idwa.org
http://www.knowledgecenters.org/dwcenter.asp
Recommended Perl Programming Links
http://language.perl.com/style/
FAQ
Please also read the DBI FAQ which is installed as a DBI::FAQ module.
You can use perldoc to read it by executing the "perldoc DBI::FAQ" com-
mand.
AUTHORS
DBI by Tim Bunce. This pod text by Tim Bunce, J. Douglas Dunlop,
Jonathan Leffler and others. Perl by Larry Wall and the
"perl5-porters".
COPYRIGHT
The DBI module is Copyright (c) 1994-2002 Tim Bunce. Ireland. All
rights reserved.
You may distribute under the terms of either the GNU General Public
License or the Artistic License, as specified in the Perl README file.
ACKNOWLEDGEMENTS
I would like to acknowledge the valuable contributions of the many peo-
ple I have worked with on the DBI project, especially in the early
years (1992-1994). In no particular order: Kevin Stock, Buzz Moschetti,
Kurt Andersen, Ted Lemon, William Hails, Garth Kennedy, Michael Pep-
pler, Neil S. Briscoe, Jeff Urlwin, David J. Hughes, Jeff Stander, For-
rest D Whitcher, Larry Wall, Jeff Fried, Roy Johnson, Paul Hudson,
Georg Rehfeld, Steve Sizemore, Ron Pool, Jon Meek, Tom Christiansen,
Steve Baumgarten, Randal Schwartz, and a whole lot more.
Then, of course, there are the poor souls who have struggled through
untold and undocumented obstacles to actually implement DBI drivers.
Among their ranks are Jochen Wiedmann, Alligator Descartes, Jonathan
Leffler, Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin Pratomo,
Davide Migliavacca, Jan Pazdziora, Peter Haworth, Edmund Mergl, Steve
Williams, Thomas Lowery, and Phlip Plumlee. Without them, the DBI would
not be the practical reality it is today. I'm also especially grateful
to Alligator Descartes for starting work on the "Programming the Perl
DBI" book and letting me jump on board.
Much of the DBI and DBD::Oracle was developed while I was Technical
Director (CTO) of the Paul Ingram Group (www.ig.co.uk). So I'd espe-
cially like to thank Paul for his generosity and vision in supporting
this work for many years.
TRANSLATIONS
A German translation of this manual (possibly slightly out of date) is
available, thanks to O'Reilly, at:
http://www.oreilly.de/catalog/perldbiger/
Some other translations:
http://cronopio.net/perl/ - Spanish
http://member.nifty.ne.jp/hippo2000/dbimemo.htm - Japanese
SUPPORT / WARRANTY
The DBI is free software. IT COMES WITHOUT WARRANTY OF ANY KIND.
Commercial support for Perl and the DBI, DBD::Oracle and Oraperl mod-
ules can be arranged via The Perl Clinic. For more details visit:
http://www.perlclinic.com
For direct DBI and DBD::Oracle support, enhancement, and related work I
am available for consultancy on standard commercial terms.
TRAINING
References to DBI related training resources. No recommendation
implied.
http://www.treepax.co.uk/
http://www.keller.com/dbweb/
FREQUENTLY ASKED QUESTIONS
See the DBI FAQ for a more comprehensive list of FAQs. Use the "perldoc
DBI::FAQ" command to read it.
How fast is the DBI?
To measure the speed of the DBI and DBD::Oracle code, I modified
DBD::Oracle so you can set an attribute that will cause the same row to
be fetched from the row cache over and over again (without involving
Oracle code but exercising *all* the DBI and DBD::Oracle code in the
code path for a fetch).
The results (on my lightly loaded old Sparc 10) fetching 50000 rows
using:
1 while $csr->fetch;
were: one field: 5300 fetches per cpu second (approx) ten
fields: 4000 fetches per cpu second (approx)
Obviously results will vary between platforms (newer faster platforms
can reach around 50000 fetches per second), but it does give a feel for
the maximum performance: fast. By way of comparison, using the code:
1 while @row = $csr->fetchrow_array;
("fetchrow_array" is roughly the same as "ora_fetch") gives:
one field: 3100 fetches per cpu second (approx)
ten fields: 1000 fetches per cpu second (approx)
Notice the slowdown and the more dramatic impact of extra fields. (The
fields were all one char long. The impact would be even bigger for
longer strings.)
Changing that slightly to represent actually doing something in Perl
with the fetched data:
while(@row = $csr->fetchrow_array) {
$hash{++$i} = [ @row ];
}
gives: ten fields: 500 fetches per cpu second (approx)
That simple addition has *halved* the performance.
I therefore conclude that DBI and DBD::Oracle overheads are small com-
pared with Perl language overheads (and probably database overheads).
So, if you think the DBI or your driver is slow, try replacing your
fetch loop with just:
1 while $csr->fetch;
and time that. If that helps then point the finger at your own code. If
that doesn't help much then point the finger at the database, the plat-
form, the network etc. But think carefully before pointing it at the
DBI or your driver.
(Having said all that, if anyone can show me how to make the DBI or
drivers even more efficient, I'm all ears.)
Why doesn't my CGI script work right?
Read the information in the references below. Please do not post CGI
related questions to the dbi-users mailing list (or to me).
http://www.perl.com/cgi-bin/pace/pub/doc/FAQs/cgi/perl-cgi-faq.html
http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
http://www.boutell.com/faq/
http://www.perl.com/perl/faq/
General problems and good ideas:
Use the CGI::ErrorWrap module.
Remember that many env vars won't be set for CGI scripts.
How can I maintain a WWW connection to a database?
For information on the Apache httpd server and the "mod_perl" module
see
http://perl.apache.org/
What about ODBC?
A DBD::ODBC module is available.
Does the DBI have a year 2000 problem?
No. The DBI has no knowledge or understanding of dates at all.
Individual drivers (DBD::*) may have some date handling code but are
unlikely to have year 2000 related problems within their code. However,
your application code which uses the DBI and DBD drivers may have year
2000 related problems if it has not been designed and written well.
See also the "Does Perl have a year 2000 problem?" section of the Perl
FAQ:
http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html
OTHER RELATED WORK AND PERL MODULES
Apache::DBI by E.Mergl@bawue.de
To be used with the Apache daemon together with an embedded Perl
interpreter like "mod_perl". Establishes a database connection
which remains open for the lifetime of the HTTP daemon. This way
the CGI connect and disconnect for every database access becomes
superfluous.
JDBC Server by Stuart 'Zen' Bishop zen@bf.rmit.edu.au
The server is written in Perl. The client classes that talk to it
are of course in Java. Thus, a Java applet or application will be
able to comunicate via the JDBC API with any database that has a
DBI driver installed. The URL used is in the form
"jdbc:dbi://host.domain.etc:999/Driver/DBName". It seems to be
very similar to some commercial products, such as jdbcKona.
Remote Proxy DBD support
As of DBI 1.02, a complete implementation of a DBD::Proxy driver
and the DBI::ProxyServer are part of the DBI distribution.
SQL Parser
See also the SQL::Statement module, SQL parser and engine.
perl v5.8.0 2002-12-01 DBI(3)
See also Bundle::DBI(3): man 3 Bundle::DBI
See also DBI::Const::GetInfo::ANSI(3): man 3 DBI::Const::GetInfo::ANSI
See also DBI::Const::GetInfo::ODBC(3): man 3 DBI::Const::GetInfo::ODBC
See also DBI::Const::GetInfoReturn(3): man 3 DBI::Const::GetInfoReturn
See also DBI::Const::GetInfoType(3): man 3 DBI::Const::GetInfoType
See also DBI::DBD(3): man 3 DBI::DBD
See also DBI::FAQ(3): man 3 DBI::FAQ
See also DBI::Format(3): man 3 DBI::Format
See also DBI::Profile(3): man 3 DBI::Profile
See also DBI::ProfileData(3): man 3 DBI::ProfileData
See also DBI::ProfileDumper(3): man 3 DBI::ProfileDumper
See also DBI::ProfileDumper::Apache(3): man 3 DBI::ProfileDumper::Apache
See also DBI::ProxyServer(3): man 3 DBI::ProxyServer
See also DBI::PurePerl(3): man 3 DBI::PurePerl
See also DBI::Shell(3): man 3 DBI::Shell
See also DBI::W32ODBC(3): man 3 DBI::W32ODBC
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