DBI(3) User Contributed Perl Documentation DBI(3)
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);
$ary_ref = $dbh->selectrow_arrayref($statement);
@row_ary = $dbh->selectrow_array($statement);
$hash_ref = $dbh->selectrow_hashref($statement);
$sth = $dbh->prepare($statement);
$sth = $dbh->prepare_cached($statement);
$rv = $sth->bind_param($p_num, $bind_value);
$rv = $sth->bind_param($p_num, $bind_value, $bind_type);
$rv = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute;
$rv = $sth->execute(@bind_values);
$rc = $sth->bind_col($col_num, \$col_variable);
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
@row_ary = $sth->fetchrow_array;
$ary_ref = $sth->fetchrow_arrayref;
$hash_ref = $sth->fetchrow_hashref;
$ary_ref = $sth->fetchall_arrayref;
$ary_ref = $sth->fetchall_arrayref( { ... } );
$ary_ref = $sth->fetchall_arrayref( [ ... ] );
$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 = $dbh->disconnect;
Thissynopsisaboveonlyliststhemajormethods.GETTINGHELP
If you have questions about DBI, you can get help from the
mailing list. You can subscribe to the
list by emailing:
Also worth a visit is the DBI home page at:
http://dbi.perl.org/
Before asking any questions, reread this document, consult
the archives and read the DBI FAQ. The archives are listed
at the end of this document and on the DBI home page. The
FAQ is installed as a DBI::FAQ module so you can read it
by executing "perldoc DBI::FAQ".
Please note that Tim Bunce does not maintain the mailing
lists or the web page (generous volunteers do that). So
please don't send mail directly to him; he just doesn't
have the time to answer questions personally. The dbi-users mailing list has lots of experienced people who
should be able to help you if you need it.
NOTE
This is the DBI specification that corresponds to the DBI
version 1.21 ("$Date: 2002/02/07 03:00:53 $").
The DBI specification is evolving at a steady pace, so
it's important to check that you have the latest copy.
The significant user-visible changes in each release are
documented in the DBI::FAQ module so you can read them by
executing "perldoc DBI::Changes".
Note also that whenever the DBI changes, the drivers take
some time to catch up. Recent versions of the DBI have
added new features (generally marked NEW in the text) that
may not yet be supported by the drivers you use. Talk to
the authors of those drivers if you need the new features.
Extensions to the DBI use the "DBIx::*" namespace. See
"Naming Conventions and Name Space" and:
http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
DESCRIPTION
The DBI is a database access module for the Perl program-
ming language. It defines a set of methods, variables,
and conventions that provide a consistent database inter-
face, independent of the actual database being used.
It is important to remember that the DBI is just an inter-
face. The DBI is a layer of "glue" between an application
and one or more database driver modules. It is the driver
modules which do most of the real work. The DBI provides a
standard interface and framework for the drivers to oper-
ate within.
ArchitectureofaDBIApplication
||
.-. .--------------. .-------------.
.-------. | |---| 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 functions of the corresponding
database engine. Only authors of sophisti-
cated/multi-database applications or generic library func-
tions need be concerned with drivers.
NotationandConventions
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.
OutlineUsage
To use DBI, first you need to load the DBI module:
use DBI;
use strict;
(The "use strict;" isn't required but is strongly recom-
mended.)
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 con-
nect at the start of your program and disconnect at the
end.
Explicitly defining the required "AutoCommit" behavior is
strongly recommended and may become mandatory in a later
version. This determines whether changes are automati-
cally 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" state-
ment is:
prepare,
execute,
execute,
execute.
for example:
$sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");
while() {
chomp;
my ($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;
GeneralInterfaceRules&Caveats
The DBI does not have a concept of a "current session".
Every session has a handle object (i.e., a $dbh) returned
from the "connect" method. That handle object is used to
invoke database related methods.
Most data is returned to the Perl script as strings. (Null
values are returned as "undef".) This allows arbitrary
precision numeric data to be handled without loss of accu-
racy. Beware that Perl may not preserve the same accuracy
when the string is used as a number.
Dates and times are returned as character strings in the
current default format of the corresponding database
engine. Time zone effects are database/driver dependent.
Perl supports binary data in Perl strings, and the DBI
will pass binary data to and from the driver without
change. It is up to the driver implementors to decide how
they wish to handle such binary data.
Most databases that understand multiple character sets
have a default global charset. Text stored in the database
is, or should be, stored in that charset; if not, then
that's the fault of either the database or the application
that inserted the data. When text is fetched it should be
automatically converted to the charset of the client, pre-
sumably based on the locale. If a driver needs to set a
flag to get that behavior, then it should do so; it should
not require the application to do that.
Multiple SQL statements may not be combined in a single
statement handle ($sth), although some databases and
drivers do support this (notably Sybase and SQL Server).
Non-sequential record reads are not supported in this ver-
sion 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".
NamingConventionsandNameSpace
The DBI package and all packages below it ("DBI::*") are
reserved for use by the DBI. Extensions and related mod-
ules 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 sig-
nify 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).
Driver Specific Prefix Registry:
ad_ DBD::AnyData
ado_ DBD::ADO
best_ DBD::BestWins
csv_ DBD::CSV
db2_ DBD::DB2
f_ DBD::File
file_ DBD::TextFile
ib_ DBD::InterBase
ing_ DBD::Ingres
ix_ DBD::Informix
msql_ DBD::mSQL
mysql_ DBD::mysql
odbc_ DBD::ODBC
ora_ DBD::Oracle
pg_ DBD::Pg
proxy_ DBD::Proxy
rdb_ DBD::RDB
sapdb_ DBD::SAP_DB
solid_ DBD::Solid
syb_ DBD::Sybase
tdat_ DBD::Teradata
tuber_ DBD::Tuber
uni_ DBD::Unify
xbase_ DBD::XBase
SQL-AQueryLanguage
Most DBI drivers require applications to use a dialect of
SQL (Structured Query Language) to interact with the
database engine. The following links provide useful
information and further links about SQL:
http://www.altavista.com/query?q=sql+tutorial
http://www.jcc.com/sql_stnd.html
http://www.contrib.andrew.cmu.edu/~shadow/sql.html
The DBI itself does not mandate or require any particular
language to be used; it is language independent. In ODBC
terms, the DBI is in "pass-thru" mode, although individual
drivers might not be. The only requirement is that queries
and other statements must be expressed as a single string
of characters passed as the first argument to the "pre-
pare" or "do" methods.
For an interesting diversion on the real history of RDBMS
and SQL, from the people who made it happen, see:
http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html
Follow the "And the rest" and "Intergalactic dataspeak"
links for the SQL history.
PlaceholdersandBindValues
Some drivers support placeholders and bind values. Place-holders, also called parameter markers, are used to indi-
cate values in a database statement that will be supplied
later, before the prepared statement is executed. For
example, an application might use the following to insert
a row of data into the SALES table:
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
or the following, to select the description for a product:
SELECT description FROM products WHERE product_code = ?
The "?" characters are the placeholders. The association
of actual values with placeholders is known as binding,
and the values are referred to as bindvalues.
When using placeholders with the SQL "LIKE" qualifier, you
must remember that the placeholder substitutes for the
whole string. So you should use ""... LIKE ? ..."" and
include any wildcard characters in the value that you bind
to the placeholder.
NullValues
Undefined values, or "undef", can be used to indicate null
values. However, care must be taken in the particular
case of trying to use null values to qualify a "SELECT"
statement. Consider:
SELECT description FROM products WHERE product_code = ?
Binding an "undef" (NULL) to the placeholder will not
select rows which have a NULL "product_code"! Refer to the
SQL manual for your database engine or any SQL book for
the reasons for this. To explicitly select NULLs you have
to say ""WHERE product_code IS NULL"" and to make that
general you have to say:
... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL))
and bind the same value to both placeholders.
Performance
Without using placeholders, the insert statement shown
previously would have to contain the literal values to be
inserted and would have to be re-prepared and re-executed
for each row. With placeholders, the insert statement only
needs to be prepared once. The bind values for each row
can be given to the "execute" method each time it's
called. By avoiding the need to re-prepare the statement
for each row, the application 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 state-
ment. Use the double-quote like "qq{...}" operator if you
want to interpolate variables into the string. See "Quote
and Quote-like Operators" in perlop for more details.
See also the "bind_column" method, which is used to asso-
ciate Perl variables with the output columns of a "SELECT"
statement.
THEDBIPACKAGEANDCLASS
In this section, we cover the DBI class methods, utility
functions, and the dynamic attributes associated with
generic DBI handles.
DBIConstants
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) omitted, 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 sup-
port that data type.
DBIClassMethods
The following methods are provided by the DBI class:
"connect"
$dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect($data_source, $username, $password, \%attr)
or die $DBI::errstr;
Establishes a database connection, or session, to the
requested $data_source. Returns a database handle
object if the connection succeeds. Use "$dbh-">"dis-
connect" to terminate the connection.
If the connect fails (see below), it returns "undef"
and sets both $DBI::err and $DBI::errstr. (It does not
set $!, etc.) You should generally test the return
status of "connect" and "print $DBI::errstr" if it has
failed.
Multiple simultaneous connections to multiple
databases through multiple drivers can be made via the
DBI. Simply make one "connect" call for each database
and keep a copy of each returned database handle.
The $data_source value should begin with
""dbi:"driver_name":"". The driver_name specifies the
driver that will be used to make the connection. (Let-
ter case is significant.)
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 pre-
fix 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 nostandard 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 $data_source is not ""Proxy"") then
the connect request will automatically be changed to:
dbi:Proxy:$ENV{DBI_AUTOPROXY};dsn=$data_source
and passed to the DBD::Proxy module. "DBI_AUTOPROXY"
is typically set as ""hostname=...;port=..."". 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 vari-
ables, respectively. The DBI will warn if the envi-
ronment 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.
"DBI-">"connect" automatically installs the driver if
it has not been installed yet. Driver installation
either returns a valid driver handle, or it dies with
an error message that includes the string
""install_driver"" and the underlying problem. So
"DBI-">"connect" will die on a driver installation
failure and will only return "undef" on a connect
failure, in which case $DBI::errstr will hold the
error message.
The $data_source argument (with the ""dbi:...:"" pre-
fix removed) and the $username and $password arguments
are then passed to the driver for processing. The DBI
does not define any interpretation for the contents of
these fields. The driver is free to interpret the
$data_source, $username, and $password fields in any
way, and supply whatever defaults are appropriate for
the engine being accessed. (Oracle, for example, uses
the ORACLE_SID and TWO_TASK environment variables if
no $data_source is specified.)
The "AutoCommit" and "PrintError" attributes for each
connection default to "on". (See "AutoCommit" and
"PrintError" for more information.) However, it is
strongly recommended that you explicitly define "Auto-
Commit" rather than rely on the default. Future ver-
sions of the DBI may issue a warning if "AutoCommit"
is not explicitly defined.
The "\%attr" parameter can be used to alter the
default settings of "PrintError", "RaiseError", "Auto-
Commit", and other attributes. For example:
$dbh = DBI->connect($data_source, $user, $pass, {
PrintError => 0,
AutoCommit => 0
});
You can also define connection attribute values within
the $data_source parameter. For example:
dbi:DriverName(PrintError=>0,Taint=>1):...
Individual attributes values specified in this way
take precedence over any conflicting values specified
via the "\%attr" parameter to "connect".
The "dbi_connect_method" attribute can be used to
specify which driver method should be called to estab-
lish the connection. The only useful values are 'con-
nect', 'connect_cached', or some specialized case like
'Apache::DBI::connect' (which is automatically the
default when running within Apache).
Where possible, each session ($dbh) is independent
from the transactions in other sessions. This is use-
ful when you need to hold cursors open across transac-
tions--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 argu-
ment to "connect" (instead of "\%attr"):
$dbh = DBI->connect($data_source, $user, $pass, $driver);
In this "old-style" form of "connect", the
$data_source should not start with
""dbi:driver_name:"". (If it does, the embedded
driver_name will be ignored). Also note that in this
older form of "connect", the "$dbh-">"{AutoCommit}"
attribute is undefined, the "$dbh-">"{PrintError}"
attribute is off, and the old "DBI_DBNAME" environment
variable is checked if "DBI_DSN" is not defined.
Beware that this "old-style" "connect" will be with-
drawn in a future version of DBI.
"connect_cached" NEW
$dbh = DBI->connect_cached($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
or die $DBI::errstr;
"connect_cached" is like "connect", except that the
database handle returned is also stored in a hash
associated with the given parameters. If another call
is made to "connect_cached" with the same parameter
values, then the corresponding cached $dbh will be
returned if it is still valid. The cached database
handle is replaced with a new connection if it has
been disconnected or if the "ping" method fails.
Note that the behavior of this method differs in sev-
eral respects from the behavior of presistent connec-
tions implemented by Apache::DBI.
Caching can be useful in some applications, but it can
also cause problems and should be used with care. The
exact behavior of this method is liable to change, so
if you intend to use it in any production applications
you should discuss your needs on the dbi-users mailing
list.
The cache can be accessed (and cleared) via the
"CachedKids" attribute.
"available_drivers"
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($quiet);
Returns a list of all available drivers by searching
for "DBD::*" modules through the directories in @INC.
By default, a warning is given if some drivers are
hidden by others of the same name in earlier directo-
ries. Passing a true value for $quiet will inhibit the
warning.
"data_sources"
@ary = DBI->data_sources($driver);
@ary = DBI->data_sources($driver, \%attr);
Returns a list of all data sources (databases) avail-
able via the named driver. If $driver is empty or
"undef", then the value of the "DBI_DRIVER" environ-
ment variable is used.
The driver will be loaded if it hasn't been already.
Note that if the driver loading fails then it dies
with an error message that includes the string
""install_driver"" and the underlying problem.
Data sources are returned in a form suitable for pass-
ing to the "connect" method (that is, they will
include the ""dbi:$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, such as a connected
database handle, to be supplied.
"trace"
DBI->trace($trace_level)
DBI->trace($trace_level, $trace_filename)
DBI trace information can be enabled for all handles
using the "trace" DBI class method. To enable trace
information for a specific handle, use the similar
"$h-">"trace" method described elsewhere.
Trace levels are as follows:
0 - Trace disabled.
1 - Trace DBI method calls returning with results or errors.
2 - Trace method entry with parameters and returning with results.
3 - As above, adding some high-level information from the driver
and some internal information from the DBI.
4 - As above, adding more detailed information from the driver.
Also includes DBI mutex information when using threaded Perl.
5 and above - As above but with more and more obscure information.
Trace level 1 is best for a simple overview of what's
happening. Trace level 2 is a good choice for general
purpose tracing. Levels 3 and above (up to 9) are
best reserved for investigating a specific problem,
when you need to see "inside" the driver and DBI.
The trace output is detailed and typically very use-
ful. Much of the trace output is formatted using the
"neat" function, so strings in the trace output may be
edited and truncated.
Initially trace output is written to "STDERR". If
$trace_filename is specified and can be opened in
append mode then all trace output (including that from
other handles) is redirected to that file. A warning
is generated is the file can't be opened. Further
calls to "trace" without a $trace_filename do not
alter where the trace output is sent. If $trace_file-
name is undefined, then trace output is sent to
"STDERR" and the previous trace file is closed. The
"trace" method returns the previous tracelevel.
See also the "$h-">"trace" and "$h-">"trace_msg" meth-
ods and the "DEBUGGING" section for information about
the "DBI_TRACE" environment variable.
DBIUtilityFunctions
In addition to the methods listed in the previous section,
the DBI package also provides these utility functions:
"neat"
$str = DBI::neat($value, $maxlen);
Return a string containing a neat (and tidy) represen-
tation 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). 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 internally by the DBI for
"trace" output. It should typically not be used for
formatting values for database use. (See also
"quote".)
"neat_list"
$str = DBI::neat_list(\@listref, $maxlen, $field_sep);
Calls "DBI::neat" on each element of the list and
returns a string containing the results joined with
$field_sep. $field_sep defaults to ", ".
"looks_like_number"
@bool = DBI::looks_like_number(@array);
Returns true for each element that looks like a num-
ber. Returns false for each element that does not
look like a number. Returns "undef" for each element
that is undefined or empty.
"hash"
$hash_value = DBI::hash($buffer, $type);
Return a 32-bit integer 'hash' value corresponding to
the contents of $buffer. The $type parameter selects
which kind of hash 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 nega-
tive (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.
DBIDynamicAttributes
Dynamic attributes are always associated with the lasthandleused (that handle is represented by $h in the
descriptions below).
Where an attribute is equivalent to a method call, then
refer to the method call for all related documentation.
Warning: these attributes are provided as a convenience
but they do have limitations. Specifically, they have a
short lifespan: because they are associated with the last
handle used, they should only be used immediately after
calling the method that "sets" them. If in any doubt, use
the corresponding method call.
$DBI::err
Equivalent to "$h-">"err".
$DBI::errstr
Equivalent to "$h-">"errstr".
$DBI::state
Equivalent to "$h-">"state".
$DBI::rows
Equivalent to "$h-">"rows". Please refer to the docu-
mentation 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.
METHODSCOMMONTOALLHANDLES
The following methods can be used by all types of DBI han-
dles.
"err"
$rv = $h->err;
Returns the native database engine error code from the
last driver method called. The code is typically an
integer but you should not assume that.
The DBI resets $h->err to undef before most DBI method
calls, so the value only has a short lifespan. Also,
most drivers share the same error variables across all
their handles, so calling a method on one handle will
typically reset the error on all the other handles
that are children of that driver.
If you need to test for individual errors and have
your program be portable to different database
engines, then you'll need to determine what the corre-
sponding error codes are for all those engines and
test for all of them.
"errstr"
$str = $h->errstr;
Returns the native database engine error message from
the last driver method called. This has the same
lifespan issues as the "err" method described above.
"state"
$str = $h->state;
Returns an error code in the standard SQLSTATE five
character format. Note that the specific success code
00000 is translated to '' (false). If the driver does
not support SQLSTATE (and most don't), then state will
return "S1000" (General Error) for all errors.
The driver is free to return any value via "state",
e.g., warning codes, even if it has not declared an
error by returning a true value via the "err" method
described above.
"set_err" NEW
$rv = $h->set_err($err, $errstr);
$rv = $h->set_err($err, $errstr, $state, $method);
$rv = $h->set_err($err, $errstr, $state, $method, $rv);
Set the "err", "errstr", and "state" values for the
handle. This will trigger the normal DBI error han-
dling mechanisms, such as "RaiseError" and "HandleEr-
ror", if they are enabled. This method is typically
only used by DBI drivers and DBI subclasses.
The $method parameter provides an alternate method
name, instead of the fairly unhelpful '"set_err"', for
the "RaiseError"/"PrintError" error string.
The "set_err" method normally returns undef. The $rv
parameter provides an alternate return value. The
"HandleError" subroutine can access and alter this
value.
"trace"
$h->trace($trace_level);
$h->trace($trace_level, $trace_filename);
DBI trace information can be enabled for a specific
handle (and any future children of that handle) by
setting the trace level using the "trace" method.
Trace level 1 is best for a simple overview of what's
happening. Trace level 2 is a good choice for general
purpose tracing. Levels 3 and above (up to 9) are
best reserved for investigating a specific problem,
when you need to see "inside" the driver and DBI. Set
$trace_level to 0 to disable the trace.
The trace output is detailed and typically very use-
ful. Much of the trace output is formatted using the
"neat" function, so strings in the trace output may be
edited and truncated.
Initially, trace output is written to "STDERR". If
$trace_filename is specified, then the file is opened
in append mode and all trace output (including that
from other handles) is redirected to that file. Fur-
ther calls to trace without a $trace_filename do not
alter where the trace output is sent. If $trace_file-
name is undefined, then trace output is sent to
"STDERR" and the previous trace file is closed.
See also the "DBI-">"trace" method, the
"$h-">"{TraceLevel}" attribute, and "DEBUGGING" for
information about the "DBI_TRACE" environment vari-
able.
"trace_msg"
$h->trace_msg($message_text);
$h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if trace is
enabled for $h or for the DBI as a whole. Can also be
called as "DBI-">"trace_msg($msg)". See "trace".
If $min_level is defined, then the message is output
only if the trace level is equal to or greater than
that level. $min_level defaults to 1.
"func"
$h->func(@func_arguments, $func_name);
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.
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.
ATTRIBUTESCOMMONTOALLHANDLES
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 state-
ment handle do not affect the parent 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
is fatal, except for private driver specific attributes
(which all have names starting with a lowercase letter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
"Warn" (boolean, inherited)
Enables useful warnings for certain bad practices.
Enabled by default. Some emulation layers, especially
those for Perl 4 interfaces, disable warnings. Since
warnings are generated using the Perl "warn" function,
they can be intercepted using the Perl $SIG{__WARN__}
hook.
"Active" (boolean, read-only)
True if the handle object is "active". This is rarely
used in applications. The exact meaning of active is
somewhat vague at the moment. For a database handle it
typically means that the handle is connected to a
database ("$dbh-">"disconnect" sets "Active" off).
For a statement handle it typically means that the
handle is a "SELECT" that may have more data to fetch.
(Fetching all the data or calling "$sth-">"finish"
sets "Active" off.)
"Kids" (integer, read-only)
For a driver handle, "Kids" is the number of currently
existing database handles that were created from that
driver handle. For a database handle, "Kids" is the
number of currently existing statement handles that
were created from that database handle.
"ActiveKids" (integer, read-only)
Like "Kids", but only counting those that are "Active"
(as above).
"CachedKids" (hash ref)
For a database handle, returns a reference to the
cache (hash) of statement handles created by the "pre-
pare_cached" method. For a driver handle, returns a
reference to the cache (hash) of database handles cre-
ated by the "connect_cached" method.
"CompatMode" (boolean, inherited)
Used by emulation layers (such as Oraperl) to enable
compatible behavior in the underlying driver (e.g.,
DBD::Oracle) for this handle. Not normally set by
application code.
"InactiveDestroy" (boolean)
This attribute can be used to disable the databaseengine related effect of DESTROYing a handle (which
would normally close a prepared statement or discon-
nect from the database etc).
For a database handle, this attribute does not disable
an explicit call to the disconnect method, only the
implicit call from DESTROY.
The default value, false, means that a handle will be
automatically destroyed when it passes out of scope.
A true value disables automatic destruction. (Think of
the name as meaning 'inactive 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 "InactiveDestroy" on all their shared handles.
Note that some databases, including Oracle, don't sup-
port passing a database connection across a fork.
"PrintError" (boolean, inherited)
This attribute can be used to force errors to generate
warnings (using "warn") in addition to returning error
codes in the normal way. When set "on", any method
which results in an error occuring will cause the DBI
to effectively do a "warn("$class $method failed:
$DBI::errstr")" where $class is the driver class and
$method is the name of the method which failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
By default, "DBI-">"connect" sets "PrintError" "on".
If desired, the warnings can be caught and processed
using a $SIG{__WARN__} handler or modules like
CGI::Carp and CGI::ErrorWrap.
"RaiseError" (boolean, inherited)
This attribute can be used to force errors to raise
exceptions rather than simply return error codes in
the normal way. It is "off" by default. When set
"on", any method which results in an error will cause
the DBI to effectively do a "die("$class $method
failed: $DBI::errstr")", where $class is the driver
class and $method is the name of the method that
failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If you turn "RaiseError" on then you'd normally turn
"PrintError" off. If "PrintError" is also on, then
the "PrintError" is done first (naturally).
Typically "RaiseError" is used in conjunction with
"eval { ... }" to catch the exception that's been
thrown and followed by an "if ($@) { ... }" block to
handle the caught exception. In that eval block the
$DBI::lasth variable can be useful for diagnosis and
reporting. For example, $DBI::lasth->{Type} and
$DBI::lasth->{Statement}.
If you want to temporarily turn "RaiseError" off
(inside a library function that is likely to fail, for
example), the recommended way is like this:
{
local $h->{RaiseError}; # localize and turn off for this block
...
}
The original value will automatically and reliably be
restored by Perl, regardless of how the block is
exited. The same logic applies to other attributes,
including "PrintError".
Sadly, this doesn't work for Perl versions up to and
including 5.004_04. Even more sadly, for Perl 5.5 and
5.6.0 it does work but leaks memory! For backwards
compatibility, you could just use "eval { ... }"
instead.
"HandleError" (code ref, inherited) NEW
This attribute can be used to provide your own alter-
native behaviour in case of errors. If set to a refer-
ence 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 "PrintEr-
ror" 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 get 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 han-
dlers 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 important. See perlsub and
perlref for more information about closures.
It is 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(0,""); # turn off the error
$_[2] = [ ... ]; # supply alternative return value
return 1;
};
This only works for methods which return a single
value and is hard to make reliable (avoiding infinite
loops, for example) and so isn't recommended for gen-
eral use! If you find a good use for it then please
let me know.
"ShowErrorStatement" (boolean, inherited) NEW
This attribute can be used to cause the relevant
Statement text to be appended to the error messages
generated by the "RaiseError" and "PrintError"
attributes. Only applies to errors on statement han-
dles plus the prepare() and do() database handle meth-
ods. (The exact format of the appended text is sub-
ject to change.)
"TraceLevel" (integer, inherited) NEW
This attribute can be used as an alternative to the
"trace" method to set the DBI trace level for a spe-
cific handle.
"FetchHashKeyName" (string, inherited)
This attribute is used to specify which attribute name
the fetchrow_hashref() method should use to get the
field names for the hash keys. For historical reasons
it defaults to '"NAME"' but it is recommended to set
it to '"NAME_lc"' or '"NAME_uc"' according to your
preference. It can only be set for driver and database
tables. For statement handles the value is frozen
when prepare() is called.
"ChopBlanks" (boolean, inherited)
This attribute can be used to control the trimming of
trailing space characters from fixed width character
(CHAR) fields. No other field types are affected, even
where field values have trailing spaces.
The default is false (although it is possible that the
default may change). Applications that need specific
behavior should set the attribute as needed. Emulation
interfaces should set the attribute to match the
behavior of the interface they are emulating.
Drivers are not required to support this attribute,
but any driver which does not support it must arrange
to return "undef" as the attribute value.
"LongReadLen" (unsigned integer, inherited)
This attribute may be used to control the maximum
length of long fields ("blob", "memo", etc.) which the
driver will read from the database automatically when
it fetches each row of data. The "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 "prepare"'d will typically
have no effect, so it's common to set "LongReadLen" on
the $dbh before calling "prepare".
Note that the value used here has a direct effect on
the memory used by the application, so don't be too
generous.
See "LongTruncOk" for more information on truncation
behavior.
"LongTruncOk" (boolean, inherited)
This attribute may be used to control the effect of
fetching a long field value which has been truncated
(typically because it's longer than the value of the
"LongReadLen" attribute).
By default, "LongTruncOk" is false and so fetching a
long value that needs to be truncated will cause the
fetch to fail. (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".
"Taint" (boolean, inherited)
If this attribute is set to a true value and Perl is
running in taint mode (e.g., started with the "-T"
option), then all data fetched from the database is
tainted, and the arguments to most DBI method calls
are checked for being tainted. Thismaychange.
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
Taint attribute, for that statement handle, for the
duration of the fetch loop.
Currently only fetched data is tainted. It is possible
that the results of other DBI method calls, and the
value of fetched attributes, may also be tainted in
future versions. That change may well break your
applications unless you take great care now. If you
use DBI Taint mode, please report your experience and
any suggestions for changes.
"private_your_module_name_*"
The DBI provides a way to store extra information in a
DBI handle as "private" attributes. The DBI will allow
you to store and retreive any attribute which has a
name starting with ""private_"".
It is strongly recommended that you use just one pri-
vate 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} = { ... };
DBIDATABASEHANDLEOBJECTS
This section covers the methods and attributes associated
with database handles.
DatabaseHandleMethods
The following methods are specified for DBI database han-
dles:
"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 applicable, or not available.
This method is typically most useful for non-"SELECT"
statements that either cannot be prepared in advance
(due to a limitation of the driver) or do not need to
be executed repeatedly. It should not be used for
"SELECT" statements because it does not return a
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{...}" opera-
tor if you want to interpolate variables into the
string. See "Quote and Quote-like Operators" in per-
lop for more details.
"selectrow_array"
@row_ary = $dbh->selectrow_array($statement);
@row_ary = $dbh->selectrow_array($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_array" into a single call. If called in a
list context, it returns the first row of data from
the statement. If called in a scalar context, it
returns the first field of the first row. The $state-
ment parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped.
If any method fails, and "RaiseError" is not set,
"selectrow_array" will return an empty list.
In a scalar context, "selectrow_array" returns the
value of the first field. An "undef" is returned if
there are no matching rows or an error occurred. Since
that "undef" can't be distinguished from an "undef"
returned because the first field value was NULL, call-
ing "selectrow_array" in a scalar context should be
used with caution.
"selectrow_arrayref"
$ary_ref = $dbh->selectrow_array($statement);
$ary_ref = $dbh->selectrow_array($statement, \%attr);
$ary_ref = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_arrayref" into a single call. It returns the
first row of data from the statement. The $statement
parameter can be a previously prepared statement han-
dle, 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 han-
dle, 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 single call. It returns a
reference to an array containing a reference to an
array for each row of data fetched.
The $statement parameter can be a previously prepared
statement handle, in which case the "prepare" is
skipped. This is recommended if the statement is going
to be executed many times.
If "RaiseError" is not set and any method except
"fetchall_arrayref" fails then "selectall_arrayref"
will return "undef"; if "fetchall_arrayref" fails then
it will return with whatever data has been fetched
thus far. You should check "$sth-">"err" afterwards
(or use the "RaiseError" attribute) to discover if the
data is complete or was truncated due to an error.
The "fetchall_arrayref" method called by "selec-
tall_arrayref" supports a $slice parameter. You can
specify a value for $slice by including a '"Slice"' or
'"Columns"' attribute in \%attr. The only difference
between the two is that if "Slice" is not defined and
"Columns" is an array ref, then the array is assumed
to contain column index values (which count from 1),
rather than perl array index values. In which case
the array is copied and each value decremented before
passing to "/fetchall_arrayref".
"selectall_hashref"
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchall_hashref" into a single call. It returns a
reference to a hash containing one entry for each row.
The key for each row entry is specified by $key_field.
The value is a reference to a hash returned by
"fetchrow_hashref".
The $statement parameter can be a previously prepared
statement handle, in which case the "prepare" is
skipped. This is recommended if the statement is going
to be executed many times.
If any method except "fetchrow_hashref" fails, and
"RaiseError" is not set, "selectall_hashref" will
return "undef". If "fetchrow_hashref" fails and
"RaiseError" is not set, then it will return with
whatever data it has fetched thus far. $DBI::err
should be checked to catch that.
"selectcol_arrayref"
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute", and
fetching one column from all the rows, into a single
call. It returns a 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 col-
umn, or even multiple columns per row, into the result
array. This behaviour can be specified via a
'"Columns"' attribute which must be a ref to an array
containing the column number or numbers to use. For
example:
# get array of id and name pairs:
my $ary_ref = $dbh->selectcol_arrayref("select id, name from table", { Columns=>[1,2] });
my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name
"prepare"
$sth = $dbh->prepare($statement) or die $dbh->errstr;
$sth = $dbh->prepare($statement, \%attr) or die $dbh->errstr;
Prepares a single statement for later execution by the
database engine and returns a reference to a statement
handle object.
The returned statement handle can be used to get
attributes of the statement and invoke the "execute"
method. See "Statement Handle Methods".
Drivers for engines without the concept of preparing a
statement will typically just store the statement in
the returned handle and process it when "$sth-">"exe-
cute" is called. Such drivers are unlikely to give
much useful information about the statement, such as
"$sth-">"{NUM_OF_FIELDS}", until after "$sth-">"exe-
cute" 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 "Place-
holders"). 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, $allow_active)
Like "prepare" except that the statement handle
returned will be stored in a hash associated with the
$dbh. If another call is made to "prepare_cached" with
the same $statement and %attr values, then the corre-
sponding cached $sth will be returned without contact-
ing the database server.
This caching can be useful in some applications, but
it can also cause problems and should be used with
care. If the cached $sth being returned is active
(i.e., is a "SELECT" that may still have data to be
fetched) then a warning will be generated and "finish"
will be called for you. The warning can be suppressed
by setting $allow_active to true. The cache can be
accessed (and cleared) via the "CachedKids" attribute.
Here are some examples of "prepare_cached":
sub insert_hash {
my ($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);
}
"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" warn-
ing.
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 disconnect-
ing.
The transaction behavior of the "disconnect" method
is, sadly, undefined. Some database systems (such as
Oracle and Ingres) will automatically commit any out-
standing changes, but others (such as Informix) will
rollback any outstanding changes. Applications not
using "AutoCommit" should explicitly call "commit" or
"rollback" before calling "disconnect".
The database is automatically disconnected by the
"DESTROY" method if still connected when there are no
longer any references to the handle. The "DESTROY"
method for each driver should implicitly call "roll-
back" to undo any uncommitted changes. This is vital
behavior to ensure that incomplete transactions don't
get committed simply because Perl calls "DESTROY" on
every object before exiting. Also, do not rely on the
order of object destruction during "global destruc-
tion", 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 discon-
necting.
If you disconnect from a database while you still have
active statement handles, you will get a warning. The
statement handles should either be cleared (destroyed)
before disconnecting, or the "finish" method should be
called on each one.
"ping"
$rc = $dbh->ping;
Attempts to determine, in a reasonably efficient way,
if the database server is still running and the con-
nection to it is still working. Individual drivers
should implement this function in the most suitable
manner for their database engine.
The current default implementation always returns true
without actually doing anything. Actually, it returns
""0 but true"" which is true but zero. That way you
can tell if the return value is 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" NEWWarning: This method is experimental and may change.
$value = $dbh->get_info( $info_type );
Returns information about the implementation, i.e.
driver and data source capabilities, restrictions etc.
It returns "undef" for unknown or unimplemented infor-
mation types. For example:
$database_version = $dbh->get_info( 18 ); # SQL_DBMS_VER
$max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT
See "Standards Reference Information" for more
detailed information about the information types and
their meanings and possible return values.
The DBI curently doesn't provide a name to number map-
ping for the information type codes or the results.
Applications are expected to use the integer values
directly, with the name in a comment, or define their
own named values using something like the constant
pragma.
Because some DBI methods make use of get_info(),
drivers are strongly encouraged to support atleast
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" NEWWarning: This method is experimental and may change.
$sth = $dbh->table_info( $catalog, $schema, $table, $type );
$sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );
$sth = $dbh->table_info( \%attr ); # old style
Returns an active statement handle that can be used to
fetch information about tables and views that exist in
the database.
The old style interface passes all the parameters as a
reference to an attribute hash with some or all of the
following attributes:
%attr = (
TABLE_CAT => $catalog # String value of the catalog name
, TABLE_SCHEM => $schema # String value of the schema name
, TABLE_NAME => $table # String value of the table name
, TABLE_TYPE => $type # String value of the table type(s)
);
The old style interface is deprecated and will be
removed in a future version.
The support for the selection criteria is driver spe-
cific. If the driver doesn't support one or more of
them then you may get back more than you asked for and
can do the filtering yourself.
The arguments $catalog, $schema and $table may accept
search patterns according to the database/driver, for
example: $table = '%FOO%'; Remember that the under-
score 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 con-
tains a list of table types.
The statement handle returned has at least the follow-
ing 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 spe-
cific 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" and "Standards Reference Informa-
tion".
"column_info" NEWWarning: This method is experimental and may change.
$sth = $dbh->column_info( $catalog, $schema, $table, $column );
Returns an active statement handle that can be used to
fetch 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 follow-
ing 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 appli-
cable 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 column.
ORDINAL_POSITION: The column sequence number (starting
with 1).
IS_NULLABLE: Indicates if the column can accept NULLs.
Possible values are: 'NO', 'YES' and ''.
SQL/CLI defines the following additional columns:
CHAR_SET_CAT
CHAR_SET_SCHEM
CHAR_SET_NAME
COLLATION_CAT
COLLATION_SCHEM
COLLATION_NAME
UDT_CAT
UDT_SCHEM
UDT_NAME
DOMAIN_CAT
DOMAIN_SCHEM
DOMAIN_NAME
SCOPE_CAT
SCOPE_SCHEM
SCOPE_NAME
MAX_CARDINALITY
DTD_IDENTIFIER
IS_SELF_REF
Drivers capable of supplying any of those values
should do so in the corresponding column and supply
undef values for the others.
Drivers wishing to provide extra database/driver spe-
cific 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, SQL-
Columns provides more metadata.
See also "Standards Reference Information".
"primary_key_info" NEWWarning: This method is experimental and may change.
$sth = $dbh->primary_key_info( $catalog, $schema, $table );
Returns an active statement handle that can be used to
fetch information about columns that make up the pri-
mary key for a table. The arguments don't accept
search patterns (unlike table_info()).
For example:
$sth = $dbh->primary_key_info( undef, $user, 'foo' );
$data = $sth->fetchall_arrayref;
Note: The support for the selection criteria, such as
$catalog, is driver specific. If the driver doesn't
support catalogs and/or schemas, it may ignore these
criteria.
The statement handle returned has at least the follow-
ing 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 "Standards Reference Information".
"primary_key" NEWWarning: This method is experimental and may change.
@key_column_names = $dbh->primary_key( $catalog, $schema, $table );
Simple interface to the primary_key_info() method.
Returns a list of the column names that comprise the
primary key of the specified table. The list is in
primary key column sequence order.
"foreign_key_info" NEWWarning: This method is experimental and may change.
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table );
Returns an active statement handle that can be used to
fetch information about foreign keys in and/or refer-
encing the specified table(s). The arguments don't
accept search patterns (unlike table_info()).
$pk_catalog, $pk_schema, $pk_table identify the pri-
mary (unique) key table (PKT).
$fk_catalog, $fk_schema, $fk_table identify the for-
eign 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 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.
PKTABLE_SCHEM(UK_TABLE_SCHEM): The primary
(unique) key table schema identifier. 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 appli-
cable to the table.
FKTABLE_SCHEM(FK_TABLE_SCHEM): The foreign key
table schema identifier. This field is NULL ("undef")
if not applicable to the data source, and empty if not
applicable to the table.
FKTABLE_NAME(FK_TABLE_NAME): The foreign key
table 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 deferrabil-
ity 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 alternate key
and PRIMARY if the foreign key references a primary
key, or it may be undefined if the driver doesn't have
access to the information.
See also "Standards Reference Information".
"tables" NEWWarning: This method is experimental and may change.
@names = $dbh->tables( $catalog, $schema, $table, $type );
@names = $dbh->tables; # deprecated
Simple interface to table_info(). Returns a list of
matching table names, possibly including a cata-
log/schema prefix.
See "table_info" for a description of the parameters.
If "$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.
"type_info_all"
Warning: This method is experimental and may change.
$type_info_all = $dbh->type_info_all;
Returns a reference to an array which holds informa-
tion about each data type variant supported by the
database and driver. The array and its contents should
be treated as read-only.
The first item is a reference to an 'index' hash of
"Name ="> "Index" pairs. The items following that are
references to arrays, one per supported data type
variant. The leading index hash defines the names and
order of the fields within the arrays that follow it.
For example:
$type_info_all = [
{ TYPE_NAME => 0,
DATA_TYPE => 1,
COLUMN_SIZE => 2, # was PRECISION originally
LITERAL_PREFIX => 3,
LITERAL_SUFFIX => 4,
CREATE_PARAMS => 5,
NULLABLE => 6,
CASE_SENSITIVE => 7,
SEARCHABLE => 8,
UNSIGNED_ATTRIBUTE=> 9,
FIXED_PREC_SCALE => 10, # was MONEY originally
AUTO_UNIQUE_VALUE => 11, # was AUTO_INCREMENT originally
LOCAL_TYPE_NAME => 12,
MINIMUM_SCALE => 13,
MAXIMUM_SCALE => 14,
NUM_PREC_RADIX => 15,
},
[ 'VARCHAR', SQL_VARCHAR,
undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
],
[ 'INTEGER', SQL_INTEGER,
undef, "", "", undef,0, 0,1,0,0,0,undef,0, 0, 10
],
];
Note that more than one row may have the same value in
the "DATA_TYPE" field if there are different ways to
spell the type name and/or there are variants of the
type with different attributes (e.g., with and without
"AUTO_UNIQUE_VALUE" set, with and without
"UNSIGNED_ATTRIBUTE", etc).
The rows are ordered by "DATA_TYPE" first and then by
how closely each type maps to the corresponding ODBC
SQL data type, closest first.
The meaning of the fields is described in the documen-
tation for the "type_info" method. The index values
shown above (e.g., "NULLABLE ="> 6) are for illustra-
tion only. Drivers may define the fields with a dif-
ferent order.
This method is not normally used directly. The
"type_info" method provides a more useful interface to
the data.
Even though an 'index' hash is provided, all the field
names in the index hash defined above will always have
the index values defined above. This is defined
behaviour so that you don't need to rely on the index
hash, which is handy because the lettercase of the
keys is not defined. It is usually uppercase, as show
here, but drivers are free to return names with any
lettercase. Drivers are also free to return extra
driver-specific columns of information - though it's
recommended that they start at column index 50 to
leave room for expansion of the DBI/ODBC specifica-
tion.
"type_info"
Warning: This method is experimental and may change.
@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information
about one or more variants of $data_type. The list is
ordered by "DATA_TYPE" first and then by how closely
each type maps to the corresponding ODBC SQL data
type, closest first. If called in a scalar context
then only the first (best) element is returned.
If $data_type is undefined or "SQL_ALL_TYPES", then
the list will contain hashes for all data type vari-
ants 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 con-
ventions as the rest of the DBI (see "Naming Conven-
tions and Name Space"). The following items should
exist:
TYPE_NAME (string)
Data type name for use in CREATE TABLE statements
etc.
DATA_TYPE (integer)
SQL data type number.
COLUMN_SIZE (integer)
For numeric types, this is either the total number
of digits (if the NUM_PREC_RADIX value is 10) or
the total number of bits allowed in the column (if
NUM_PREC_RADIX is 2).
For string types, this is the maximum size of the
string in bytes.
For date and interval types, this is the maximum
number of characters needed to display the value.
LITERAL_PREFIX (string)
Characters used to prefix a literal. A typical
prefix is ""'"" for characters, or possibly ""0x""
for binary values passed as hexadecimal. NULL
("undef") is returned for data types for which
this is not applicable.
LITERAL_SUFFIX (string)
Characters used to suffix a literal. Typically
""'"" for 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 "DECIMAL" would be
""precision,scale"" if the DECIMAL type should be
declared as "DECIMAL("precision,scale")" where
precision and scale are integer values. For a
"VARCHAR" it would be ""max length"". NULL
("undef") is returned for data types for which
this is not applicable.
NULLABLE (integer)
Indicates whether the data type accepts a NULL
value: 0 or an empty string = no, 1 = yes, 2 =
unknown.
CASE_SENSITIVE (boolean)
Indicates whether the data type is case sensitive
in collations and comparisons.
SEARCHABLE (integer)
Indicates how the data type can be used in a WHERE
clause, as follows:
0 - Cannot be used in a WHERE clause
1 - Only with a LIKE predicate
2 - All comparison operators except LIKE
3 - Can be used in a WHERE clause with any comparison operator
UNSIGNED_ATTRIBUTE (boolean)
Indicates whether the data type is unsigned. NULL
("undef") is returned for data types for which
this is not applicable.
FIXED_PREC_SCALE (boolean)
Indicates whether the data type always has the
same precision and scale (such as a money type).
NULL ("undef") is returned for data types for
which this is not applicable.
AUTO_UNIQUE_VALUE (boolean)
Indicates whether a column of this data type is
automatically set to a unique value whenever a new
row is inserted. NULL ("undef") is returned for
data types for which this is not applicable.
LOCAL_TYPE_NAME (string)
Localized version of the "TYPE_NAME" for use in
dialog with users. NULL ("undef") is returned if
a localized name is not available (in which case
"TYPE_NAME" should be used).
MINIMUM_SCALE (integer)
The minimum scale of the data type. If a data type
has a fixed scale, then "MAXIMUM_SCALE" holds the
same value. NULL ("undef") is returned for data
types for which this is not applicable.
MAXIMUM_SCALE (integer)
The maximum scale of the data type. If a data type
has a fixed scale, then "MINIMUM_SCALE" holds the
same value. NULL ("undef") is returned for data
types for which this is not applicable.
SQL_DATA_TYPE (integer)
This column is the same as the "DATA_TYPE" column,
except for interval and datetime data types. For
interval and datetime data types, the
"SQL_DATA_TYPE" field will return "SQL_INTERVAL"
or "SQL_DATETIME", and the "SQL_DATETIME_SUB"
field below will return the subcode for the spe-
cific interval or datetime data type. If this
field is NULL, then the driver does not support or
report on interval or date subtypes.
SQL_DATETIME_SUB (integer)
For interval or datetime data types, where the
"SQL_DATA_TYPE" field above is "SQL_INTERVAL" or
"SQL_DATETIME", this field will hold the subcode
for the specific interval or datetime data type.
Otherwise it will be NULL ("undef").
NUM_PREC_RADIX (integer)
The radix value of the data type. For approximate
numeric types, "NUM_PREC_RADIX" contains the value
2 and "COLUMN_SIZE" holds the number of bits. For
exact numeric types, "NUM_PREC_RADIX" contains the
value 10 and "COLUMN_SIZE" holds the number of
decimal digits. NULL ("undef") is returned either
for data types for which this is not applicable or
if the driver cannot report this information.
INTERVAL_PRECISION (integer)
The interval leading precision for interval types.
NULL is returned either for data types for which
this is not applicable or if the driver cannot
report this information.
For example, to find the type name for the fields in a
select statement you can do:
@names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{TYPE} }
Since DBI and ODBC drivers vary in how they map their
types into the ISO standard types you may need to
search for more than one type. Here's an example
looking for a usable type to store a date:
$my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );
Similarly, to more reliably find a type to store small
integers, you could use a list starting with
"SQL_SMALLINT", "SQL_INTEGER", "SQL_DECIMAL", etc.
See also "Standards Reference Information".
"quote"
$sql = $dbh->quote($value);
$sql = $dbh->quote($value, $data_type);
Quote a string literal for use as a literal value in
an SQL statement, by escaping any special characters
(such as quotation marks) contained within the string
and adding the required type of outer quotation marks.
$sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
$dbh->quote("Don't");
For most database types, quote would return 'Don''t'
(including the outer quotation marks).
An undefined $value value will be returned as the
string "NULL" (without single quotation marks) to
match how NULLs are represented in SQL.
If $data_type is supplied, it is used to try to deter-
mine the required quoting behavior by using the infor-
mation returned by "type_info". As a special case,
the standard numeric types are optimized to return
$value without calling "type_info".
Quote will probably not be able to deal with all pos-
sible input (such as binary data or data containing
newlines), and is not related in any way with escaping
or quoting shell meta-characters. There is no need to
quote values being used with "Placeholders and Bind
Values".
"quote_identifier"
$sql = $dbh->quote_identifier( $name );
$sql = $dbh->quote_identifier( $name1, $name2, $name3, \%attr );
Quote an identifier (table name etc.) for use in an
SQL statement, by escaping any special characters
(such as double quotation marks) it contains and
adding the required type of outer quotation marks.
Undefined names are ignored and the remainder are
quoted and then joined together, typically with a dot
(.) character. For example:
$id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );
would, for most database types, return "Her
schema"."My table" (including all the double quotation
marks).
If three names are supplied then the first is assumed
to be a catalog name and special rules may be applied
based on what "get_info" returns for SQL_CATA-
LOG_NAME_SEPARATOR (41) and SQL_CATALOG_LOCATION
(114). For example, for Oracle:
$id = $dbh->quote_identifier( 'link', 'schema', 'table' );
would return "schema"."table"@"link".
DatabaseHandleAttributes
This section describes attributes specific to database
handles.
Changes to these database handle attributes do not affect
any other existing or future database handles.
Attempting to set or get the value of an unknown attribute
is fatal, except for private driver-specific attributes
(which all have names starting with a lowercase letter).
Example:
$h->{AutoCommit} = ...; # set/write
... = $h->{AutoCommit}; # get/read
"AutoCommit" (boolean)
If true, then database changes cannot be rolled-back
(undone). If false, then database changes automati-
cally occur within a "transaction", which must either
be committed or rolled back using the "commit" or
"rollback" methods.
Drivers should always default to "AutoCommit" mode (an
unfortunate choice largely forced on the DBI by ODBC
and JDBC conventions.)
Attempting to set "AutoCommit" to an unsupported value
is a fatal error. This is an important feature of the
DBI. Applications that need full transaction behavior
can set "$dbh-">"{AutoCommit} = 0" (or set "AutoCom-
mit" to 0 via "connect") without having to check that
the value was assigned successfully.
For the purposes of this description, we can divide
databases into three categories:
Databases which don't support transactions at all.
Databases in which a transaction is always active.
Databases in which a transaction must be explicitly started (C).
*Databaseswhichdon'tsupporttransactionsatall
For these databases, attempting to turn "AutoCommit"
off is a fatal error. "commit" and "rollback" both
issue warnings about being ineffective while "AutoCom-
mit" is in effect.
*Databasesinwhichatransactionisalwaysactive
These are typically mainstream commercial relational
databases with "ANSI standard" transaction behavior.
If "AutoCommit" is off, then changes to the database
won't have any lasting effect unless "commit" is
called (but see also "disconnect"). If "rollback" is
called then any changes since the last commit are
undone.
If "AutoCommit" is on, then the effect is the same as
if the DBI called "commit" automatically after every
successful database operation. In other words, calling
"commit" or "rollback" explicitly while "AutoCommit"
is on would be ineffective because the changes would
have already been commited.
Changing "AutoCommit" from off to on should issue a
"commit" in most drivers.
Changing "AutoCommit" from on to off should have no
immediate effect.
For databases which don't support a specific auto-com-
mit mode, the driver has to commit each statement
automatically using an explicit "COMMIT" after it com-
pletes successfully (and roll it back using an
explicit "ROLLBACK" if it fails). The error informa-
tion reported to the application will correspond to
the statement which was executed, unless it succeeded
and the commit or rollback failed.
*Databasesinwhichatransactionmustbeexplicitlystarted
For these databases, the intention is to have them act
like databases in which a transaction is always active
(as described above).
To do this, the DBI driver will automatically begin a
transaction when "AutoCommit" is turned off (from the
default "on" state) and will automatically begin
another transaction after a "commit" or "rollback".
In this way, the application does not have to treat
these databases as a special case.
See "commit", "disconnect" and "Transactions" for
other important notes about transactions.
"Driver" (handle)
Holds the handle of the parent driver. The only recom-
mended use for this is to find the name of the driver
using:
$dbh->{Driver}->{Name}
"Name" (string)
Holds the "name" of the database. Usually (and recom-
mended to be) the same as the ""dbi:DriverName:...""
string used to connect to the database, but with the
leading ""dbi:DriverName:"" removed.
"Statement" (string, read-only)
Returns the statement string passed to the most recent
"prepare" method called in this database handle, even
if that method failed. This is especially useful where
"RaiseError" is enabled and the exception handler
checks $@ and sees that a 'prepare' method call
failed.
"RowCacheSize" (integer)
A hint to the driver indicating the size of the local
row cache that the application would like the driver
to use for future "SELECT" statements. If a row cache
is not implemented, then setting "RowCacheSize" is
ignored and getting the value returns "undef".
Some "RowCacheSize" values have special meaning, as
follows:
0 - Automatically determine a reasonable cache size for each C
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