NAME

       Lire::Aggregate - Base class for all operators that compute values from a group of DLF
       records

SYNOPSIS

           use Lire::Aggregate;

           use base qw/ Lire::Aggregate /;

DESCRIPTION

       The Lire::Aggregate is the base class of all operators that computes values from a group
       of DLF records. These are the operators that are used inside aggregators.

       This package also defines most of the classes that represents the group operators
       available in Lire report specification.

METHODS FOR SUBCLASSES OF Aggregate

   init( %params )
       Two supplemental parameters are needed for Aggregates:

       parent
           The Lire::Aggregator which contain this Aggregate.

       name
           The identifier that can be used to find this operator in the report specification. It
           will be returned by the name() method.

   print( $fh, $prefix)
       This methods implements the print() method required by Lire::ReportOperator. It prints an
       empty XML element named after op().  It also takes care of writing the name and label
       attributes. Other attributes can be added to the XML element by overriding the xml_attrs()
       method.

   xml_attrs()
       This method can be used to write additional XML attributes. The returned string will be
       output in the XML element.

   set_name( $name )
       Changes this operator's name.

   create_numerical_info( $group_info )
       Subclass must implement this method.

       The $group_info parameter is a Lire::Report::GroupInfo object to which the operator
       implementation should add the appropriate ColumnInfo object to represent the data
       generated by the aggregate.

   build_query( $query )
       FIXME.

       Provides a default implementation of build_query

   set_missing_cases_aggr_expr( $query )
       Adds the aggregate expression to compute the number of missing cases based on the required
       fields returned by the sql_required_fields() method.

       The method set_missing_cases_value() can be used to set the value from the returned DLF
       row.

   set_missing_cases_value( $row, $value )
       Sets the missing-cases value in $value to the value computed by the expression sets using
       set_missing_cases_aggr_expr().

   sql_aggr_expr()
       Method used by the default implementation of build_query(). It should return the SQL
       agregate expression that should be associated to this Aggregate's name. For example, a
       simple Count implementation could have returned 'count(*)' here.

   sql_required_fields()
       Method used by the set_missing_cases_aggr_expr() to sets an aggregate expression that will
       return the number of missing cases for this aggregate. It should returns a list of fields
       that will create a missing case when a value is NULL.

   create_value( $parent_group, $row )
       Method that is used by Aggregator when building the report table. The report should return
       an hash reference with the appropriate keys set.  Consult Lire::Report::Entry(3pm) for
       details.

       The $parent_group parameter is the Lire::Report::Group to which the value is added. This
       will be the table or containing group for values added to Entries, and it will be the
       Group's parent when creating a summary value.

       The $row parameter is an hash ref containing the DlfResult row for which the value should
       be created.

       Implementation should use the set_missing_cases_value() method to set the value's
       'missing_cases' attribute.

SEE ALSO

       Lire::ReportSpec(3pm), Lire::ReportOperator(3pm), Lire::Aggregator(3pm).

AUTHORS

         Francis J. Lacoste <flacoste@logreport.org>
         Wolfgang Sourdeau <wolfgang@logreport.org>

VERSION

       $Id: Aggregate.pm,v 1.16 2006/07/23 13:16:27 vanbaal Exp $

COPYRIGHT

       Copyright (C) 2001-2003 Stichting LogReport Foundation LogReport@LogReport.org

       This file is part of Lire.

       Lire is free software; you can redistribute it and/or modify it under the terms of the GNU
       General Public License as published by the Free Software Foundation; either version 2 of
       the License, or (at your option) any later version.

       This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
       without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU General Public License for more details.

       You should have received a copy of the GNU General Public License along with this program
       (see COPYING); if not, check with http://www.gnu.org/copyleft/gpl.html.