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SQL::Eval - Base for deriving evalution objects for SQL::Statement


    require SQL::Statement;
    require SQL::Eval;

    # Create an SQL statement; use a concrete subclass of
    # SQL::Statement
    my $stmt = MyStatement->new("SELECT * FROM foo, bar",

    # Get an eval object by calling open_tables; this
    # will call MyStatement::open_table
    my $eval = $stmt->open_tables($data);

    # Set parameter 0 to 'Van Gogh'
    $eval->param(0, 'Van Gogh');
    # Get parameter 2
    my $param = $eval->param(2);

    # Get the SQL::Eval::Table object referring the 'foo' table
    my $fooTable = $eval->table('foo');


This module implements two classes that can be used for deriving concrete subclasses to evaluate SQL::Statement objects. The SQL::Eval object can be thought as an abstract state engine for executing SQL queries, the SQL::Eval::Table object can be considered a *very* table abstraction. It implements method for fetching or storing rows, retrieving column names and numbers and so on. See the test.pl script as an example for implementing a concrete subclass.

While reading on, keep in mind that these are abstract classes, you *must* implement at least some of the methods describe below. Even more, you need not derive from SQL::Eval or SQL::Eval::Table, you just need to implement the method interface.

All methods just throw a Perl exception in case of errors.

Method interface of SQL::Eval


Constructor; use it like this:

    $eval = SQL::Eval->new(\%attr);

Blesses the hash ref \%attr into the SQL::Eval class (or a subclass).


Used for getting or setting input parameters, as in the SQL query

    INSERT INTO foo VALUES (?, ?);


    $eval->param(0, $val);        # Set parameter 0
    $eval->param(0);              # Get parameter 0

Likewise used for getting or setting the complete array of input parameters. Example:

    $eval->params($params);       # Set the array
    $eval->params();              # Get the array

Returns or sets a table object. Example:

    $eval->table('foo', $fooTable);  # Set the 'foo' table object
    $eval->table('foo');             # Return the 'foo' table object

Return the value of a column with a given name; example:

    $col = $eval->column('foo', 'id');  # Return the 'id' column of
                                        # the current row in the
                                        # 'foo' table

This is equivalent and just a shorthand for

    $col = $eval->table('foo')->column('id');

Method interface of SQL::Eval::Table


Constructor; use it like this:

    $eval = SQL::Eval::Table->new(\%attr);

Blesses the hash ref \%attr into the SQL::Eval::Table class (or a subclass).


Used to get the current row as an array ref. Do not mismatch getting the current row with the fetch_row method! In fact this method is valid only after a successfull $table->fetchrow(). Example:

    $row = $table->row();

Get the column with a given name in the current row. Valid only after a successfull $table->fetchrow(). Example:

    $col = $table->column($colName);

Return the number of the given column name. Column numbers start with 0. Returns undef, if a column name is not defined, so that you can well use this for verifying valid column names. Example:

    $colNum = $table->column_num($colNum);

Returns an array ref of column names.

The above methods are implemented by SQL::Eval::Table. The following methods aren't, so that they *must* be implemented by concrete subclassed. See the test.pl script for example.


Fetches the next row from the table. Returns undef, if the last row was already fetched. The argument $data is for private use of the concrete subclass. Example:

    $row = $table->fetch_row($data);

Note, that you may use

    $row = $table->row();

for retrieving the same row again, until the next call of fetch_row.


Likewise for storing rows. Example:

    $table->push_row($data, $row);

Used by the CREATE TABLE statement to set the column names of the new table. Receives an array ref of names. Example:

    $table->push_names($data, $names);

Similar to the seek method of a filehandle; used for setting the number of the next row being written. Example:

    $table->seek($data, $whence, $rowNum);

Actually the current implementation is using only seek($data, 0,0) (first row) and seek($data, 2,0) (last row, end of file).


Truncates a table after the current row. Example:



The current implementation is quite simple: An SQL::Eval object is an hash ref with only two attributes. The params attribute is an array ref of parameters. The tables attribute is an hash ref of table names (keys) and table objects (values).

SQL::Eval::Table instances are implemented as hash refs. Used attributes are row (the array ref of the current row), col_nums (an hash ref of column names as keys and column numbers as values) and col_names, an array ref of column names with the column numbers as indexes.


All methods are working with instance-local data only, thus the module is reentrant and thread safe, if you either don't share handles between threads or grant serialized use.


This module is Copyright (C) 1998 by

    Jochen Wiedmann
    Am Eisteich 9
    72555 Metzingen

    Email: joe@ispsoft.de
    Phone: +49 7123 14887

All rights reserved.

You may distribute this module under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.