DBI Database Handle Objects (Programming the Perl DBI)

This section covers the methods and attributes associated with database handles.

A.6.1. Database Handle Methods

The following methods are specified for DBI database handles:

$rc  = $dbh->do($statement)           || die $dbh->errstr;
$rc  = $dbh->do($statement, \%attr)   || die $dbh->errstr;
$rv  = $dbh->do($statement, \%attr, @bind_values) || ...

Prepares and executes 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 or is 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') || 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 the section on "Quote and Quote-Like Operators" in the perlop manpage 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 $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.

In a scalar context, selectrow_array returns the value of the first field. An undef is returned if there are no matching rows or if an error occurred. Since that undef can't be distinguished from an undef returned because the first field value was NULL, calling selectrow_array in a scalar context should be used with caution.

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 any method except fetch fails, and RaiseError is not set, selectall_arrayref will return undef. If fetch fails, and RaiseError is not set, then it will return with whatever data it has fetched thus far.

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.

prepare

$sth = $dbh->prepare($statement)          || die $dbh->errstr;
$sth = $dbh->prepare($statement, \%attr)  || die $dbh->errstr;

Prepares a single statement for later execution by the database engine and returns a reference to a statement handle object.

The returned statement handle can be used to get attributes of the statement and invoke the execute method. See "Statement Handle Methods."

Drivers for engines without the concept of preparing a statement will typically just store the statement in the returned handle and process it when $sth->execute is called. Such drivers are unlikely to give much useful information about the statement, such as $sth->{NUM_OF_FIELDS}, until after $sth->execute has been called. Portable applications should take this into account.

In general, DBI drivers do not parse the contents of the statement (other than simply counting any placeholders). The statement is passed directly to the database engine, sometimes known as pass-thru mode. This has advantages and disadvantages. On the plus side, you can access all the functionality of the engine being used. On the downside, you're limited if you're using a simple engine, and you need to take extra care if you're 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 corresponding cached $sth will be returned without contacting the database server.

This caching can be useful in some applications, but it can also cause problems and should be used with care. A warning will be generated if the cached $sth being returned is active (i.e., it is a SELECT that may still have data to be fetched). This warning can be suppressed by setting $allow_active to true. The cache can be accessed (and cleared) via the CachedKids attribute.

Here's an example of one possible use of prepare_cached:

while ( ($field, $value) = each %search_fields ) {
    push @sql,   "$field = ?";
    push @values, $value;
}
$qualifier = "";
$qualifier = "where ".join(" and ", @sql) if @sql;
$sth = $dbh->prepare_cached("SELECT * FROM table $qualifier");
$sth->execute(@values);

commit

$rc  = $dbh->commit     || die $dbh->errstr;

Commits (makes 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.

rollback

$rc  = $dbh->rollback   || die $dbh->errstr;

Rolls back (undoes) 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.

disconnect

$rc = $dbh->disconnect  || warn $dbh->errstr;

Disconnects the database from the database handle. disconnect is typically used only before exiting the program. The handle is of little use after disconnecting.

The transaction behavior of the disconnect method is, sadly, undefined. Some database systems (such as Oracle and Ingres) will automatically commit any outstanding changes, but others (such as Informix) will roll back any outstanding changes. Applications not using AutoCommit should explicitly call commit or rollback before calling disconnect.

The database is automatically disconnected by the DESTROY method if still connected when there are no longer any references to the handle. The DESTROY method for each driver should implicitly call rollback to undo any uncommitted changes. This is vital behavior to ensure that incomplete transactions don't get committed simply because Perl calls DESTROY on every object before exiting. Also, do not rely on the order of object destruction during "global destruction," as it is undefined.

Generally, if you want your changes to be commited or rolled back when you disconnect, then you should explicitly call commit or rollback before disconnecting.

If you disconnect from a database while you still have active statement handles, 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 connection to it is still working. Individual drivers should implement this function in the most suitable manner for their database engine.

The current default implementation always returns true without actually doing anything. Actually, it returns "0 but true", which is true but zero. That way you can tell if the return value is 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 use for this method. See the specialized Apache::DBI module for one example usage.

table_info (NEW )

$sth = $dbh->table_info;

Warning: This method is experimental and may change.

Returns an active statement handle that can be used to fetch information about tables and views that exist in the database.

The handle 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.

For more detailed information about the fields and their meanings, refer to:

http://msdn.microsoft.com/library/psdk/dasdk/odch6wqb.htm

If that URL ceases to work, then use the MSDN search facility at:

http://search.microsoft.com/us/dev/

and search for SQLTables returns using the exact phrase option. The link you want will probably just be called SQLTables and will be part of the Data Access SDK.

tables (NEW )

@names = $dbh->tables;

Warning: This method is experimental and may change.

Returns a list of table and view names, possibly including a schema prefix. This list should include all tables that can be used in a SELECT statement without further qualification.

Note that table_info might not return records for all tables. Applications can use any valid table regardless of whether it's returned by tables. See also table_info.

type_info_all (NEW )

$type_info_all = $dbh->type_info_all;

Warning: This method is experimental and may change.

Returns a reference to an array that holds information about each datatype 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 a hash of Name => Index pairs. The following items are references to arrays, one per supported datatype variant. The leading hash defines the names and order of the fields within the following list of arrays. 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 datatype, closest first.

The meaning of the fields is described in the documentation for the type_info method. The index values shown above (e.g., NULLABLE => 6) are for illustration only. Drivers may define the fields with a different order.

This method is not normally used directly. The type_info method provides a more useful interface to the data.

type_info (NEW )

@type_info = $dbh->type_info($data_type);

Warning: This method is experimental and may change.

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 datatype, 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 datatype 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 items should exist:

TYPE_NAME (string): Datatype name for use in CREATE TABLE statements, etc.
DATA_TYPE (integer): SQL datatype 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 datatypes for which this is not applicable.
LITERAL_SUFFIX (string): Characters used to suffix a literal. Typically "'" for characters. NULL (undef) is returned for datatypes where this is not applicable.
CREATE_PARAMS (string): Parameters for a datatype 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 datatypes for which this is not applicable.
NULLABLE (integer): Indicates whether the datatype accepts a NULL value: 0 = no, 1 = yes, 2 = unknown.
CASE_SENSITIVE ( boolean): Indicates whether the datatype is case-sensitive in collations and comparisons.
SEARCHABLE (integer): Indicates how the datatype 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 datatype is unsigned. NULL (undef) is returned for datatypes for which this is not applicable.
FIXED_PREC_SCALE ( boolean): Indicates whether the datatype always has the same precision and scale (such as a money type). NULL (undef) is returned for datatypes for which this is not applicable.
AUTO_UNIQUE_VALUE ( boolean): Indicates whether a column of this datatype is automatically set to a unique value whenever a new row is inserted. NULL (undef) is returned for datatypes 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 datatype. If a datatype has a fixed scale, then MAXIMUM_SCALE holds the same value. NULL (undef) is returned for datatypes for which this is not applicable.
MAXIMUM_SCALE (integer): The maximum scale of the datatype. If a datatype has a fixed scale, then MINIMUM_SCALE holds the same value. NULL (undef) is returned for datatypes 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 datatypes. For interval and datetime datatypes, 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 datatype. 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 datatypes, 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 datatype. Otherwise it will be NULL (undef).
NUM_PREC_RADIX (integer): The radix value of the datatype. 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 datatypes 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 datatypes for which this is not applicable or if the driver cannot report this information.

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 ] );

For more detailed information about these fields and their meanings, refer to:

http://msdn.microsoft.com/library/psdk/dasdk/odch6yy7.htm

If that URL ceases to work, then use the MSDN search facility at:

http://search.microsoft.com/us/dev/

and search the MSDN library for SQLGetTypeInfo returns using the exact phrase option. The link you want will probably just be called SQLGetTypeInfo (there may be more than one).

The individual datatypes are currently described here:

http://msdn.microsoft.com/library/psdk/dasdk/odap8fcj.htm

If that URL ceases to work, or to get more general information, use the MSDN search facility as described above, and search for SQL Data Types.

quote

$sql = $dbh->quote($value);
$sql = $dbh->quote($value, $data_type);

Quotes 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 quotation marks) to match how NULLs are represented in SQL.

If $data_type is supplied, it is used to try to determine the required quoting behavior by using the information returned by type_info. As a special case, the standard numeric types are optimized to return $value without calling type_info.

quote will probably not be able to deal with all possible input (such as binary data or data containing newlines), and is not related in any way with escaping or quoting shell meta-characters. There is no need to quote values being used with placeholders and bind values.

A.6. DBI Database Handle Objects

A.6.1. Database Handle Methods

A.6.2. Database Handle Attributes

A.6.2.1. Databases that don't support transactions at all

A.6.2.2. Databases in which a transaction is always active

A.6.2.3. Databases in which a transaction must be explicitly started