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A.6. DBI Database Handle Objects

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:

do


$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.

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.

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.

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 ] );

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.

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.

A.6.2. Database Handle Attributes

This section describes attributes specific to database handles.

Changes to these database handle attributes do not affect any other existing or future database handles.

Attempting to set or get the value of an unknown attribute is fatal, except for private driver-specific attributes (which all have names starting with a lowercase letter).

For example:

$h->{AutoCommit} = ...;       # set/write
... = $h->{AutoCommit};       # get/read

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

For these databases, attempting to turn AutoCommit off is a fatal error. commit and rollback both issue warnings about being ineffective while AutoCommit is in effect.

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

These are typically mainstream commercial relational databases with "ANSI standard" transaction behavior. If AutoCommit is off, then changes to the database won't have any lasting effect unless 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 that don't support a specific autocommit mode, the driver has to commit each statement automatically using an explicit COMMIT after it completes successfully (and roll it back using an explicit rollback if it fails). The error information reported to the application will correspond to the statement that was executed, unless it succeeded and the commit or rollback failed.

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

For these databases, the intention is to have them act like databases in which a transaction is always active (as described earlier).

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 disconnect for other important notes about transactions.



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