DBI(3pm) User Contributed Perl Documentation DBI(3pm)
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);
$rv = $sth->execute_array(\%attr, ...);
$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;
The synopsis above only lists the major methods and parameters.
GETTING HELP
If you have questions about DBI, or DBD driver modules, you can get help from the
dbi-users AT perl.org mailing list. You can get help on subscribing and using the list by
emailing dbi-users-help AT perl.org.
(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.catb.org/~esr/faqs/smart-questions.html>)
The DBI home page at <http://dbi.perl.org/> is always worth a visit and includes an FAQ
and links to other resources.
Before asking any questions, reread this document, consult the archives and read the DBI
FAQ. The archives are listed at the end of this document and on the DBI home page. An FAQ
is installed as a DBI::FAQ module so you can read it by executing "perldoc DBI::FAQ".
However the DBI::FAQ module is currently (2004) outdated relative to the online FAQ on the
DBI home page.
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 personally. 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 1.46.
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. Newer versions of the DBI have added features that may not yet be supported by
the drivers you use. Talk to the authors of your drivers if you need a new feature that's
not yet supported.
Features added after DBI 1.21 (February 2002) are marked in the text with the version num-
ber of the DBI release they first appeared in.
Extensions to the DBI API often use the "DBIx::*" namespace. See "Naming Conventions and
Name Space". DBI extension modules can be found at
<http://search.cpan.org/search?mode=module&query=DBIx>. And all modules related to the
DBI can be found at <http://search.cpan.org/search?query=DBI&mode=all>.
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, inde-
pendent 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 frame-
work 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 interface 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 implementations for methods, and many other non-database specific
duties.
Each driver contains implementations of the DBI methods using the private interface func-
tions 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" behaviour is strongly recommended and may
become mandatory in a later version. This determines whether changes are automatically
committed to the database when executed, or need to be explicitly committed later.
The DBI allows an application to "prepare" statements for later execution. 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 $dbh) returned from the "connect" method. That handle object is used to invoke
database related methods.
Most data is returned to the Perl script as strings. (Null values are returned as
"undef".) This allows arbitrary precision numeric data to be handled without loss of
accuracy. Beware that Perl may not preserve the same accuracy when the string is used as
a number.
Dates and times are returned as character strings in the current default format of the
corresponding database engine. Time zone effects are database/driver dependent.
Perl supports binary data in Perl strings, and the DBI will pass binary data to and from
the driver without change. It is up to the driver implementors to decide how they wish to
handle such binary data.
Most databases that understand multiple character sets have a default global charset. Text
stored in the database is, or should be, stored in that charset; if not, then that's the
fault of either the database or the application that inserted the data. When text is
fetched it should be automatically converted to the charset of the client, presumably
based on the locale. If a driver needs to set a flag to get that behaviour, then it should
do so; it should not require the application to do that.
Multiple SQL statements may not be combined in a single statement handle ($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 functions 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" parameters 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::" namespace (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 lowercase 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).
SQL - A Query Language
Most DBI drivers require applications to use a dialect of SQL (Structured Query Language)
to interact with the database engine. The "Standards Reference Information" section pro-
vides links to useful information about SQL.
The DBI itself does not mandate or require any particular language to be used; it is lan-
guage 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 "prepare"
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 "Full Contents" then "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 fol-
lowing 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 placehold-
ers is known as binding, and the values are referred to as bind values.
Note that the "?" is not enclosed in quotation marks, even when the placeholder represents
a string. Some drivers also allow placeholders like ":"name and ":"n (e.g., ":1", ":2",
and so on) in addition to "?", but their use is not portable.
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
"SELECT name, age FROM people WHERE name IN (?,?)" # two names
When using placeholders with the SQL "LIKE" qualifier, you must remember that the place-
holder 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 "prod-
uct_code"! Refer to the SQL manual for your database engine or any SQL book for the rea-
sons 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 avoid-
ing the need to re-prepare the statement for each row, the application typically 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_columns" 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 produced 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) omit-
ted, because SQL/CLI and ODBC provide conflicting codes.
See the "type_info", "type_info_all", and "bind_param" methods for possible 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:
"parse_dsn"
($scheme, $driver, $attr_string, $attr_hash, $driver_dsn) = DBI->parse_dsn($dsn)
or die "Can't parse DBI DSN '$dsn'";
Breaks apart a DBI Data Source Name (DSN) and returns the individual parts. If $dsn
doesn't contain a valid DSN then parse_dsn() returns an empty list.
$scheme is the first part of the DSN and is currently always 'dbi'. $driver is the
driver name, possibly defaulted to $ENV{DBI_DRIVER}, and may be undefined.
$attr_string is the optional attribute string, which may be undefined. If
$attr_string is true then $attr_hash is a reference to a hash containing the parsed
attribute names and values. $driver_dsn is the last part of the DBI DSN string.
The parse_dsn() method was added in DBI 1.43.
"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 termi-
nate the connection.
If the connect fails (see below), it returns "undef" and sets both $DBI::err and
$DBI::errstr. (It does not explicitly set $!.) 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 must begin with ""dbi:"driver_name":"". The driver_name speci-
fies the driver that will be used to make the connection. (Letter case is signifi-
cant.)
As a convenience, if the $data_source parameter is undefined or empty, the DBI will
substitute the value of the environment variable "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 requirement 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:hostname=...;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, respec-
tively. The DBI will warn if the environment variables are not defined. However, the
everyday use of these environment variables is not recommended for security reasons.
The mechanism is primarily intended to simplify testing. See below for alternative
way to specify the username and password.
"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. Use
"eval { ... }" if you need to catch the ""install_driver"" error.
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 exam-
ple, 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 information.) However, it is strongly recom-
mended that you explicitly define "AutoCommit" rather than rely on the default. The
"PrintWarn" attribute defaults to on if $^W is true, i.e., perl is running with warn-
ings enabled.
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
});
The username and password can also be specified using the attributes "Username" and
"Password", in which case they take precedence over the $username and $password param-
eters.
You can also define connection attribute values within the $data_source parameter. For
example:
dbi:DriverName(PrintWarn=>1,PrintError=>0,Taint=>1):...
Individual attributes values specified in this way take precedence over any conflict-
ing values specified via the "\%attr" parameter to "connect".
The "dbi_connect_method" attribute can be used to specify which driver method should
be called to establish the connection. The only useful values are 'connect', 'con-
nect_cached', or some specialized case like 'Apache::DBI::connect' (which is automati-
cally the default when running within Apache).
Where possible, each session ($dbh) is independent from the transactions in other ses-
sions. This is useful when you need to hold cursors open across transactions--for
example, if you use one session 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 unde-
fined, 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" "con-
nect" will soon be withdrawn in a future version of DBI.
"connect_cached"
$dbh = DBI->connect_cached($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 parameters. If another call is made to
"connect_cached" with the same parameter values, then the corresponding cached $dbh
will be returned if it is still valid. The cached database handle is replaced with a
new connection if it has been disconnected or if the "ping" method fails.
Note that the behaviour of this method differs in several respects from the behaviour
of persistent connections implemented by Apache::DBI.
Caching connections can be useful in some applications, but it can also cause prob-
lems, such as too many connections, and so should be used with care.
The cache can be accessed (and cleared) via the "CachedKids" attribute.
"available_drivers"
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($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 oth-
ers of the same name in earlier directories. Passing a true value for $quiet will
inhibit the warning.
"installed_versions"
DBI->installed_versions;
@ary = DBI->installed_versions;
%hash = DBI->installed_versions;
Calls available_drivers() and attempts to load each of them in turn using
install_driver(). For each load that succeeds the driver name and version number are
added to a hash. When running under DBI::PurePerl drivers which appear not be pure-
perl are ignored.
When called in array context the list of successfully loaded drivers is returned
(without the 'DBD::' prefix).
When called in scalar context a reference to the hash is returned and the hash will
also contain other entries for the "DBI" version, "OS" name, etc.
When called in a void context the installed_versions() method will print out a format-
ted list of the hash contents, one per line.
Due to the potentially high memory cost and unknown risks of loading in an unknown
number of drivers that just happen to be installed on the system, this method is nor
recommended for general use. Use available_drivers() instead.
The installed_versions() method is primarily intended as a quick way to see from the
command line what's installed. For example:
perl -MDBI -e 'DBI->installed_versions'
The installed_versions() method was added in DBI 1.38.
"data_sources"
@ary = DBI->data_sources($driver);
@ary = DBI->data_sources($driver, \%attr);
Returns a list of data sources (databases) available via the named driver. If $driver
is empty or "undef", then the value of the "DBI_DRIVER" environment variable is used.
The driver will be loaded if it hasn't been already. Note that if the driver loading
fails then data_sources() dies with an error message that includes the string
""install_driver"" and the underlying problem.
Data sources are returned in a form suitable for passing to the "connect" method (that
is, they will include the ""dbi:$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 incomplete list or may require driver-specific
attributes.
There is also a data_sources() method defined for database handles.
"trace"
DBI->trace($trace_setting)
DBI->trace($trace_setting, $trace_filename)
$trace_setting = DBI->trace;
The "DBI->trace" method sets the global default trace settings and returns the
previous trace settings. It can also be used to change where the trace output is sent.
There's a similar method, "$h->trace", which sets the trace settings for the specific
handle it's called on.
See the "TRACING" section for full details about the DBI's powerful tracing facili-
ties.
DBI Utility Functions
In addition to the DBI methods listed in the previous section, the DBI package also pro-
vides several utility functions.
These can be imported into your code by listing them in the "use" statement. For example:
use DBI qw(neat data_diff);
Alternatively, all these utility functions (except hash) can be imported using the
":utils" import tag. For example:
use DBI qw(:utils);
"data_string_desc"
$description = data_string_desc($string);
Returns an informal description of the string. For example:
UTF8 off, ASCII, 42 characters 42 bytes
UTF8 off, non-ASCII, 42 characters 42 bytes
UTF8 on, non-ASCII, 4 characters 6 bytes
UTF8 on but INVALID encoding, non-ASCII, 4 characters 6 bytes
UTF8 off, undef
The initial "UTF8" on/off refers to Perl's internal SvUTF8 flag. If $string has the
SvUTF8 flag set but the sequence of bytes it contains are not a valid UTF-8 encoding
then data_string_desc() will report "UTF8 on but INVALID encoding".
The "ASCII" vs "non-ASCII" portion shows "ASCII" if all the characters in the string
are ASCII (have code points <= 127).
The data_string_desc() function was added in DBI 1.46.
"data_string_diff"
$diff = data_string_diff($a, $b);
Returns an informal description of the first character difference between the strings.
If both $a and $b contain the same sequence of characters then data_string_diff()
returns an empty string. For example:
Params a & b Result
------------ ------
'aaa', 'aaa' ''
'aaa', 'abc' 'Strings differ at index 2: a[2]=a, b[2]=b'
'aaa', undef 'String b is undef, string a has 3 characters'
'aaa', 'aa' 'String b truncated after 2 characters'
Unicode characters are reported in "\x{XXXX}" format. Unicode code points in the range
U+0800 to U+08FF are unassigned and most likely to occur due to double-encoding. Char-
acters in this range are reported as "\x{08XX}='C'" where "C" is the corresponding
latin-1 character.
The data_string_diff() function only considers logical characters and not the underly-
ing encoding. See "data_diff" for an alternative.
The data_string_diff() function was added in DBI 1.46.
"data_diff"
$diff = data_diff($a, $b);
$diff = data_diff($a, $b, $logical);
Returns an informal description of the difference between two strings. It calls
"data_string_desc" and "data_string_diff" and returns the combined results as a multi-
line string.
For example, "data_diff("abc", "ab\x{263a}")" will return:
a: UTF8 off, ASCII, 3 characters 3 bytes
b: UTF8 on, non-ASCII, 3 characters 5 bytes
Strings differ at index 2: a[2]=c, b[2]=\x{263A}
If $a and $b are identical in both the characters they contain and their physical
encoding then data_diff() returns an empty string. If $logical is true then physical
encoding differences are ignored (but are still reported if there is a difference in
the characters).
The data_diff() function was added in DBI 1.46.
"neat"
$str = neat($value);
$str = neat($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).
If the string is flagged internally as utf8 then double quotes will be used, otherwise
single quotes are used and unprintable 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 inter-
nally by the DBI for "trace" output. It should typically not be used for formatting
values for database use. (See also "quote".)
"neat_list"
$str = neat_list(\@listref, $maxlen, $field_sep);
Calls "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 = looks_like_number(@array);
Returns true for each element that looks like a number. Returns false for each ele-
ment that does not look like a number. Returns "undef" for each element that is unde-
fined 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 algorithm 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 repre-
sented 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, for most drivers, the statement handles share the same error
variable as the parent database handle, so calling a method on one handle may reset
the error on the related handles.
If you need to test for individual errors and have your program be portable to differ-
ent database engines, then you'll need to determine what the corresponding error codes
are for all those engines and test for all of them.
A driver may return 0 from err() to indicate a warning condition after a method call.
Similarly, a driver may return an empty string to indicate a 'success with informa-
tion' condition. In both these cases the value is false but not undef. The errstr()
and state() methods may be used to retrieve extra information in these cases.
See "set_err" for more information.
"errstr"
$str = $h->errstr;
Returns the native database engine error message from the last DBI method called. This
has the same lifespan issues as the "err" method described above.
The returned string may contain multiple messages separated by newline characters.
The errstr() method should not be used to test for errors, use err() for that, because
drivers may return 'success with information' or warning messages via errstr() for
methods that have not 'failed'.
See "set_err" for more information.
"state"
$str = $h->state;
Returns a state code in the standard SQLSTATE five character format. Note that the
specific success code 00000 is translated to any empty string (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.
The state() method should not be used to test for errors, use err() for that, because
drivers may return a 'success with information' or warning state code via errstr() for
methods that have not 'failed'.
"set_err"
$rv = $h->set_err($err, $errstr);
$rv = $h->set_err($err, $errstr, $state);
$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 method is typically
only used by DBI drivers and DBI subclasses.
If the "HandleSetErr" attribute holds a reference to a subroutine it is called first.
The subroutine can alter the $err, $errstr, $state, and $method values. See "Handle-
SetErr" for full details. If the subroutine returns a true value then the handle
"err", "errstr", and "state" values are not altered and set_err() returns an empty
list (it normally returns $rv which defaults to undef, see below).
Setting "err" to a true value indicates an error and will trigger the normal DBI error
handling mechanisms, such as "RaiseError" and "HandleError", if they are enabled, when
execution returns from the DBI back to the application.
Setting "err" to "" indicates an 'information' state, and setting it to "0" indicates
a 'warning' state. Setting "err" to "undef" also sets "errstr" to undef, and "state"
to "", irrespective of the values of the $errstr and $state parameters.
The $method parameter provides an alternate method name for the "RaiseError"/"PrintEr-
ror"/"PrintWarn" error string instead of the fairly unhelpful '"set_err"'.
The "set_err" method normally returns undef. The $rv parameter provides an alternate
return value.
Some special rules apply if the "err" or "errstr" values for the handle are already
set...
If "errstr" is true then: "" [err was %s now %s]"" is appended if $err is true and
"err" is already true; "" [state was %s now %s]"" is appended if $state is true and
"state" is already true; then ""\n"" and the new $errstr are appended. Obviously the
%s's above are replaced by the corresponding values.
The handle "err" value is set to $err if: $err is true; or handle "err" value is
undef; or $err is defined and the length is greater than the handle "err" length. The
effect is that an 'information' state only overrides undef; a 'warning' overrides
undef or 'information', and an 'error' state overrides anything.
The handle "state" value is set to $state if $state is true and the handle "err" value
was set (by the rules above).
Support for warning and information states was added in DBI 1.41.
"trace"
$h->trace($trace_settings);
$h->trace($trace_settings, $trace_filename);
$trace_settings = $h->trace;
The trace() method is used to alter the trace settings for a handle (and any future
children of that handle). It can also be used to change where the trace output is
sent.
There's a similar method, "DBI->trace", which sets the global default trace settings.
See the "TRACING" section for full details about the DBI's powerful tracing facili-
ties.
"trace_msg"
$h->trace_msg($message_text);
$h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if the trace level is greater than or equal to
$min_level (which defaults to 1). Can also be called as "DBI->trace_msg($msg)".
See "TRACING" for more details.
"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 status 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.)
See also "install_method" for how you can avoid needing to use func() and gain.
"can"
$is_implemented = $h->can($method_name);
Returns true if $method_name is implemented by the driver or a default method is pro-
vided by the DBI. It returns false where a driver hasn't implemented a method and the
default method is provided by the DBI is just an empty stub.
"parse_trace_flags"
$trace_settings_integer = $h->parse_trace_flags($trace_settings);
Parses a string containing trace settings and returns the corresponding integer value
used internally by the DBI and drivers.
The $trace_settings argument is a string containing a trace level between 0 and 15
and/or trace flag names separated by vertical bar (""|"") or comma ("","") characters.
For example: "SQL|3|foo".
It uses the parse_trace_flag() method, described below, to process the individual
trage flag names.
The parse_trace_flags() method was added in DBI 1.42.
"parse_trace_flag"
$bit_flag = $h->parse_trace_flag($trace_flag_name);
Returns the bit flag corresponding to the trace flag name in $trace_flag_name.
Drivers are expected to override this method and check if $trace_flag_name is a driver
specific trace flags and, if not, then call the DBIs default parse_trace_flag().
The parse_trace_flag() method was added in DBI 1.42.
"swap_inner_handle"
$rc = $h1->swap_inner_handle( $h2 );
$rc = $h1->swap_inner_handle( $h2, $allow_reparent );
Brain transplants for handles. You don't need to know about this unless you want to
become a handle surgeon.
A DBI handle is a reference to a tied hash. A tied hash has an inner hash that actu-
ally holds the contents. The swap_inner_handle() method swaps the inner hashes
between two handles. The $h1 and $h2 handles still point to the same tied hashes, but
what those hashes are tied to has been swapped. In effect $h1 becomes $h2 and
vice-versa. This is powerful stuff. Use with care.
As a small safety measure, the two handles, $h1 and $h2, have to share the same parent
unless $allow_reparent is true.
The swap_inner_handle() method was added in DBI 1.44.
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 par-
ent database handle and changes to the database handle do not affect existing statement
handles, only future ones.
Attempting to set or get the value of an unknown attribute generates a warning, except for
private driver specific attributes (which all have names starting with a lowercase let-
ter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
"Warn" (boolean, inherited)
The "Warn" attribute enables useful warnings for certain bad practices. It is enabled
by default and should only be disabled in rare circumstances. Since warnings are gen-
erated using the Perl "warn" function, they can be intercepted using the Perl
$SIG{__WARN__} hook.
The "Warn" attribute is not related to the "PrintWarn" attribute.
"Active" (boolean, read-only)
The "Active" attribute is true if the handle object is "active". This is rarely used
in applications. The exact meaning of active is somewhat vague at the moment. For a
database handle it typically means that the handle is connected to a database
("$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.)
"Executed" (boolean)
The "Executed" attribute is true if the handle object has been "executed". Currently
only the $dbh do() method and the $sth execute(), execute_array(), and exe-
cute_for_fetch() methods set the "Executed" attribute.
When it's set on a handle it is also set on the parent handle at the same time. So
calling execute() on a $sth also sets the "Executed" attribute on the parent $dbh.
The "Executed" attribute for a database handle is cleared by the commit() and roll-
back() methods. The "Executed" attribute of a statement handle is not cleared by the
DBI under any circumstances and so acts as a permanent record of whether the statement
handle was ever used.
The "Executed" attribute was added in DBI 1.41.
"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 statement handles that were created from that database handle. For
a statement handle, the value is zero.
"ActiveKids" (integer, read-only)
Like "Kids", but only counting those that are "Active" (as above).
"CachedKids" (hash ref)
For a database handle, "CachedKids" returns a reference to the cache (hash) of state-
ment 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)
The "CompatMode" attribute is used by emulation layers (such as Oraperl) to enable
compatible behaviour in the underlying driver (e.g., DBD::Oracle) for this handle. Not
normally set by application code.
It also has the effect of disabling the 'quick FETCH' of attribute values from the
handles attribute cache. So all attribute values are handled by the drivers own FETCH
method. This makes them slightly slower but is useful for special-purpose drivers like
DBD::Multiplex.
"InactiveDestroy" (boolean)
The "InactiveDestroy" attribute can be used to disable the database engine related
effect of DESTROYing a handle (which would normally close a prepared statement or dis-
connect from the database etc). The default value, false, means a handle will be
fully destroyed when it passes out of scope.
For a database handle, this attribute does not disable an explicit call to the discon-
nect method, only the implicit call from DESTROY that happens if the handle is still
marked as "Active".
Think of the name as meaning 'treat the handle as not-Active in the DESTROY method'.
This attribute is specifically designed for use in Unix applications that "fork" child
processes. Either the parent or the child process, but not both, should set "Inac-
tiveDestroy" on all their shared handles. Note that some databases, including Oracle,
don't support passing a database connection across a fork.
To help tracing applications using fork the process id is shown in the trace log when-
ever a DBI or handle trace() method is called. The process id also shown for every
method call if the DBI trace level (not handle trace level) is set high enough to show
the trace from the DBI's method dispatcher, e.g. >= 9.
"PrintWarn" (boolean, inherited)
The "PrintWarn" attribute controls the printing of warnings recorded by the driver.
When set to a true value the DBI will check method calls to see if a warning condition
has been set. If so, the DBI will effectively do a "warn("$class $method warning:
$DBI::errstr")" where $class is the driver class and $method is the name of the method
which failed. E.g.,
DBD::Oracle::db execute warning: ... warning text here ...
By default, "DBI->connect" sets "PrintWarn" "on" if $^W is true, i.e., perl is running
with warnings enabled.
If desired, the warnings can be caught and processed using a $SIG{__WARN__} handler or
modules like CGI::Carp and CGI::ErrorWrap.
See also "set_err" for how warnings are recorded and "HandleSetErr" for how to influ-
ence it.
Fetching the full details of warnings can require an extra round-trip to the database
server for some drivers. In which case the driver may opt to only fetch the full
details of warnings if the "PrintWarn" attribute is true. If "PrintWarn" is false then
these drivers should still indicate the fact that there were warnings by setting the
warning string to, for example: "3 warnings".
"PrintError" (boolean, inherited)
The "PrintError" attribute can be used to force errors to generate warnings (using
"warn") in addition to returning error codes in the normal way. When set "on", any
method which results in an error occuring will cause the DBI to effectively do a
"warn("$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 $SIG{__WARN__} handler or
modules like CGI::Carp and CGI::ErrorWrap.
"RaiseError" (boolean, inherited)
The "RaiseError" attribute can be used to force errors to raise exceptions rather than
simply return error codes in the normal way. It is "off" by default. When set "on",
any method which results in an error will cause the DBI to effectively do a
"die("$class $method failed: $DBI::errstr")", where $class is the driver class and
$method is the name of the method that failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If you turn "RaiseError" on then you'd normally turn "PrintError" off. If "PrintEr-
ror" is also on, then the "PrintError" is done first (naturally).
Typically "RaiseError" is used in conjunction with "eval { ... }" to catch the excep-
tion that's been thrown and followed by an "if ($@) { ... }" block to handle the
caught exception. In that eval block the $DBI::lasth variable can be useful for diag-
nosis 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".
"HandleError" (code ref, inherited)
The "HandleError" 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 normal.
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 "HandleError" value is impor-
tant. See perlsub and perlref for more information 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 "RaiseEr-
ror":
$h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };
It is also possible for "HandleError" to hide an error, to a limited 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(undef,undef); # 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.
"HandleSetErr" (code ref, inherited)
The "HandleSetErr" attribute can be used to intercept the setting of handle "err",
"errstr", and "state" values. If set to a reference to a subroutine then that subrou-
tine is called whenever set_err() is called, typically by the driver or a subclass.
The subroutine is called with five arguments, the first five that were passed to
set_err(): the handle, the "err", "errstr", and "state" values being set, and the
method name. These can be altered by changing the values in the @_ array. The return
value affects set_err() behaviour, see "set_err" for details.
It is possible to 'stack' multiple HandleSetErr handlers by using closures. See "Han-
dleError" for an example.
The "HandleSetErr" and "HandleError" subroutines differ in subtle but significant
ways. HandleError is only invoked at the point where the DBI is about to return to the
application with "err" set true. It's not invoked by the failure of a method that's
been called by another DBI method. HandleSetErr, on the other hand, is called when-
ever set_err() is called with a defined "err" value, even if false. So it's not just
for errors, despite the name, but also warn and info states. The set_err() method,
and thus HandleSetErr, may be called multiple times within a method and is usually
invoked from deep within driver code.
In theory a driver can use the return value from HandleSetErr via set_err() to decide
whether to continue or not. If set_err() returns an empty list, indicating that the
HandleSetErr code has 'handled' the 'error', the driver could then continue instead of
failing (if that's a reasonable thing to do). This isn't excepted to be common and
any such cases should be clearly marked in the driver documentation and discussed on
the dbi-dev mailing list.
The "HandleSetErr" attribute was added in DBI 1.41.
"ErrCount" (unsigned integer)
The "ErrCount" attribute is incremented whenever the set_err() method records an
error. It isn't incremented by warnings or information states. It is not reset by the
DBI at any time.
The "ErrCount" attribute was added in DBI 1.41. Older drivers may not have been
updated to use set_err() to record errors and so this attribute may not be incremented
when using them.
"ShowErrorStatement" (boolean, inherited)
The "ShowErrorStatement" attribute can be used to cause the relevant Statement text to
be appended to the error messages generated by the "RaiseError", "PrintError", and
"PrintWarn" attributes. Only applies to errors on statement handles plus the pre-
pare(), 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 appended to the end of the Statement text in the error
message.
"TraceLevel" (integer, inherited)
The "TraceLevel" attribute can be used as an alternative to the "trace" method to set
the DBI trace level and trace flags for a specific handle. See "TRACING" for more
details.
The "TraceLevel" attribute is especially useful combined with "local" to alter the
trace settings for just a single block of code.
"FetchHashKeyName" (string, inherited)
The "FetchHashKeyName" 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 preference. It can only be set for driver and database handles. For state-
ment handles the value is frozen when prepare() is called.
"ChopBlanks" (boolean, inherited)
The "ChopBlanks" attribute can be used to control the trimming of trailing space char-
acters 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). Applica-
tions that need specific behaviour should set the attribute as needed.
Drivers are not required to support this attribute, but any driver which does not sup-
port it must arrange to return "undef" as the attribute value.
"LongReadLen" (unsigned integer, inherited)
The "LongReadLen" 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 "LongReadLen" attribute only relates to fetching and reading long values; 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 digits. 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 "pre-
pare"'d will typically have no effect, so it's common to set "LongReadLen" on the $dbh
before calling "prepare".
For most drivers the value used here has a direct effect on the memory used by the
statement handle while it's active, so don't be too generous. If you can't be sure
what value to use you could execute an extra select statement to determine the longest
value. For example:
$dbh->{LongReadLen} = $dbh->selectrow_array{qq{
SELECT MAX(long_column_name) FROM table WHERE ...
});
$sth = $dbh->prepare(qq{
SELECT long_column_name, ... FROM table WHERE ...
});
You may need to take extra care if the table can be modified between the first select
and the second being executed.
See "LongTruncOk" for more information on truncation behaviour.
"LongTruncOk" (boolean, inherited)
The "LongTruncOk" 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 trun-
cated will cause the fetch to fail. (Applications 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
truncation, 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".
"TaintIn" (boolean, inherited)
If the "TaintIn" attribute is set to a true value and Perl is running in taint mode
(e.g., started with the "-T" option), then all the arguments 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.
The "TaintIn" attribute was added in DBI 1.31.
"TaintOut" (boolean, inherited)
If the "TaintOut" 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 con-
sidered 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 ver-
sions. 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 suggestions for changes.
The "TaintOut" attribute was added in DBI 1.31.
"Taint" (boolean, inherited)
The "Taint" attribute is a shortcut for "TaintIn" and "TaintOut" (it is also present
for backwards compatibility).
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)
The "Profile" attribute enables the collection and reporting of method call timing
statistics. See the DBI::Profile module documentation for much more detail.
The "Profile" attribute was added in DBI 1.24.
"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 retrieve 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} = { ... };
This attribute is primarily of interest to people sub-classing DBI.
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:
"clone"
$new_dbh = $dbh->clone();
$new_dbh = $dbh->clone(\%attr);
The "clone" method duplicates the $dbh connection by connecting with the same parame-
ters ($dsn, $user, $password) as originally used.
The attributes for the cloned connect are the same as those used for the original con-
nect, with some other attribute merged over them depending on the \%attr parameter.
If \%attr is given then the attributes it contains are merged into the original
attributes and override any with the same names. Effectively the same as doing:
%attribues_used = ( %original_attributes, %attr );
If \%attr is not given then it defaults to a hash containing all the attributes in the
attribute cache of $dbh excluding any non-code references, plus the main boolean
attributes (RaiseError, PrintError, AutoCommit, etc.). This behaviour is subject to
change.
The clone method can be used even if the database handle is disconnected.
The "clone" method was added in DBI 1.33. It is very new and likely to change.
"data_sources"
@ary = $dbh->data_sources();
@ary = $dbh->data_sources(\%attr);
Returns a list of data sources (databases) available via the $dbh driver's
data_sources() method, plus any extra data sources that the driver can discover via
the connected $dbh. Typically the extra data sources are other databases managed by
the same server process that the $dbh is connected to.
Data sources are returned in a form suitable for passing to the "connect" method (that
is, they will include the ""dbi:$driver:"" prefix).
The data_sources() method, for a $dbh, was added in DBI 1.38.
"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 number of rows is not known, not applica-
ble, 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
statement 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 "execute" 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.
"last_insert_id"
$rv = $dbh->last_insert_id($catalog, $schema, $table, $field);
$rv = $dbh->last_insert_id($catalog, $schema, $table, $field, \%attr);
Returns a value 'identifying' the row just inserted, if possible. Typically this
would be a value assigned by the database server to a column with an auto_increment or
serial type. Returns undef if the driver does not support the method or can't deter-
mine the value.
The $catalog, $schema, $table, and $field parameters may be required for some drivers
(see below). If you don't know the parameter values and your driver does not need
them, then use "undef" for each.
There are several caveats to be aware of with this method if you want to use it for
portable applications:
* For some drivers the value may only available immediately after the insert statement
has executed (e.g., mysql, Informix).
* For some drivers the $catalog, $schema, $table, and $field parameters are required
(e.g., Pg), for others they are ignored (e.g., mysql).
* Drivers may return an indeterminate value if no insert has been performed yet.
* For some drivers the value may only be available if placeholders have not been used
(e.g., Sybase, MS SQL). In this case the value returned would be from the last non-
placeholder insert statement.
* Some drivers may need driver-specific hints about how to get the value. For example,
being told the name of the database 'sequence' object that holds the value. Any such
hints are passed as driver-specific attributes in the \%attr parameter.
* If the underlying database offers nothing better, then some drivers may attempt to
implement this method by executing ""select max($field) from $table"". Drivers using
any approach like this should issue a warning if "AutoCommit" is true because it is
generally unsafe - another process may have modified the table between your insert and
the select. For situations where you know it is safe, such as when you have locked the
table, you can silence the warning by passing "Warn" => 0 in \%attr.
* If no insert has been performed yet, or the last insert failed, then the value is
implementation defined.
Given all the caveats above, it's clear that this method must be used with care.
The "last_insert_id" method was added in DBI 1.38.
"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 $statement 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 exer-
cise some caution if you use "selectrow_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 sin-
gle call. It returns the first row of data from the statement. The $statement parame-
ter 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 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 previously prepared statement handle, in which case the "prepare" is skipped.
If any method fails, and "RaiseError" is not set, "selectrow_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 sin-
gle 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" supports a $max_rows
parameter. You can specify a value for $max_rows by including a '"MaxRows"' attribute
in \%attr. In which case finish() is called for you after fetchall_arrayref() returns.
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, "selec-
tall_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 reference 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
You can specify a maximum number of rows to fetch by including a '"MaxRows"' attribute
in \%attr.
"prepare"
$sth = $dbh->prepare($statement) or die $dbh->errstr;
$sth = $dbh->prepare($statement, \%attr) or die $dbh->errstr;
Prepares a statement for later execution by the database engine and returns a refer-
ence to a statement handle object.
The returned statement handle can be used to get attributes of the statement and
invoke the "execute" method. See "Statement Handle Methods".
Drivers for engines without the concept of preparing a statement will typically just
store the statement in the returned handle and process it when "$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 previous statement.
Some command-line SQL tools use statement terminators, like a semicolon, 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, $if_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 parameter values, then the corresponding cached $sth will be
returned without contacting the database server.
The $if_active parameter lets you adjust the behaviour if an already cached statement
handle is still Active. There are several alternatives:
0: A warning will be generated, and finish() will be called on the statement han-
dle before it is returned. This is the default behaviour if $if_active is not
passed.
1: finish() will be called on the statement handle, but the warning is suppressed.
2: Disables any checking.
3: The existing active statement handle will be removed from the cache and a new
statement handle prepared and cached in its place. This is the safest option
because it doesn't affect the state of the old handle, it just removes it from the
cache. [Added in DBI 1.40]
Here are some examples of "prepare_cached":
sub insert_hash {
my ($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(...);
while (my $data = $sth->fetchrow_hashref) {
# later, in some other code called within the loop...
my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth2->execute(...);
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 duplicate 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.
You'll know if you run into this problem because prepare_cached() will generate a
warning by default (when $if_active is false).
The cache used by prepare_cached() is keyed by both the statement and any attributes
so you can also avoid this issue by doing something like:
$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 "rollback 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 "rollback", "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 behaviour 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. Appli-
cations 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 behaviour 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 indicate 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 imple-
ment this function in the most suitable manner for their database engine.
The current default implementation always returns true without actually doing any-
thing. Actually, it returns ""0 but true"" which is true but zero. That way you can
tell if the return value is genuine 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"
$value = $dbh->get_info( $info_type );
Returns information about the implementation, i.e. driver and data source capabili-
ties, 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 informa-
tion types and their meanings and possible return values.
The DBI::Const::GetInfoType module exports a %GetInfoType hash that can be used to map
info type names to numbers. For example:
$database_version = $dbh->get_info( $GetInfoType{SQL_DBMS_VER} );
The names are a merging of the ANSI and ODBC standards (which differ in some cases).
See DBI::Const::GetInfoType for more details.
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"
$sth = $dbh->table_info( $catalog, $schema, $table, $type );
$sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );
Returns an active statement handle that can be used to fetch information about tables
and views that exist in the database.
The arguments $catalog, $schema and $table may accept search patterns 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 optionally be quoted, e.g.:
$type = "TABLE";
$type = "'TABLE','VIEW'";
In addition the following special cases may also be supported by some drivers:
* 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('%', '', '');
* If the value of $schema is '%' and $catalog and $table are empty strings, the result
set contains a list of schema names.
* If the value of $type is '%' and $catalog, $schema, and $table are all empty
strings, the result set contains a list of table types.
If your driver doesn't support one or more of the selection filter parameters then you
may get back more than you asked for and can do the filtering yourself.
This method can be expensive, and can return a large amount of data. (For example,
small Oracle installation returns over 2000 rows.) So it's a good idea to use the
filters to limit the data as much as possible.
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"
$sth = $dbh->column_info( $catalog, $schema, $table, $column );
Returns an active statement handle that can be used to fetch information about columns
in specified tables.
The arguments $schema, $table and $column may accept search patterns 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 column 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 characters 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 col-
umn.
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 information 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 metadata.
See also "Catalog Methods" and "Standards Reference Information".
"primary_key_info"
$sth = $dbh->primary_key_info( $catalog, $schema, $table );
Returns an active statement handle that can be used to fetch information 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;
The statement handle will return one row per column, ordered by TABLE_CAT, TA-
BLE_SCHEM, TABLE_NAME, and KEY_SEQ.
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"
@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 col-
umn sequence order.
"foreign_key_info"
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table );
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table
, \%attr );
Returns an active statement handle that can be used to fetch information about foreign
keys in and/or referencing the specified table(s). The arguments don't accept search
patterns (unlike table_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 implementation-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 table catalog identi-
fier. 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.
PKTABLE_SCHEM ( UK_TABLE_SCHEM ): The primary (unique) key table schema identi-
fier. This field is NULL ("undef") if not applicable to the data source, and empty if
not applicable to the table.
PKTABLE_NAME ( UK_TABLE_NAME ): The primary (unique) key table identifier.
PKCOLUMN_NAME (UK_COLUMN_NAME ): The primary (unique) key column 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 appli-
cable to the table.
FKTABLE_NAME ( FK_TABLE_NAME ): The foreign key table identifier.
FKCOLUMN_NAME ( FK_COLUMN_NAME ): The foreign key column identifier.
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 foreign 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 alter-
nate key and PRIMARY if the foreign key references a primary key, or it may be unde-
fined if the driver doesn't have access to the information.
See also "Catalog Methods" and "Standards Reference Information".
"tables"
@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_IDENTIFIER_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. This means that the table names
returned will include quote characters.
"type_info_all"
$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,
SQL_DATA_TYPE => 15,
SQL_DATETIME_SUB => 16,
NUM_PREC_RADIX => 17,
INTERVAL_PRECISION=> 18,
},
[ '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
],
];
More than one row may have the same value in the "DATA_TYPE" field if there are dif-
ferent ways to spell the type name and/or there are variants of the type with differ-
ent 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.
An 'index' hash is provided so you don't need to rely on index values defined above.
However, using DBD::ODBC with some old ODBC drivers may return older names, shown as
comments in the example above. Another issue with the index hash is that the letter-
case of the keys is not defined. It is usually uppercase, as show here, but drivers
may 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.
The type_info_all() method is not normally used directly. The "type_info" method pro-
vides a more usable and useful interface to the data.
"type_info"
@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 following uppercase items should always
exist, though may be undef:
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 characters.
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 characters. NULL
("undef") is returned for data types where this is not applicable.
CREATE_PARAMS (string)
Parameter names for data type definition. For example, "CREATE_PARAMS" for a "DEC-
IMAL" would be ""precision,scale"" if the DECIMAL type should be declared as "DEC-
IMAL("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 "MAXI-
MUM_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 "MINI-
MUM_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 date-
time data types. For interval and datetime data types, the "SQL_DATA_TYPE" field
will return "SQL_INTERVAL" 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 support or report on interval or
datetime 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 spe-
cific interval or datetime data type. Otherwise it will be NULL ("undef").
Although not mentioned explicitly in the standards, it seems there is a simple
relationship between these values:
DATA_TYPE == (10 * SQL_DATA_TYPE) + SQL_DATETIME_SUB
NUM_PREC_RADIX (integer)
The radix value of the data type. For approximate numeric types, "NUM_P