static function createGroupBySQLStrings(&$outGroupBySelectText, &$outGroupByText, $groupBy)
 {
     if ($groupBy) {
         if (isset($groupBy['field']) and isset($groupBy['type'])) {
             $groupByField = $groupBy['field'];
             $groupByFieldType = $groupBy['type'];
             switch ($groupByField) {
                 case 'published':
                     $groupBySelect = eZContentObjectTreeNodeNoLanguage::subTreeGroupByDateField("ezcontentobject." . $groupByField, $groupByFieldType);
                     $groupBySelect['field'] = "ezcontentobject." . $groupByField;
                     break;
                 case 'modified':
                     $groupBySelect = eZContentObjectTreeNodeNoLanguage::subTreeGroupByDateField("ezcontentobject." . $groupByField, $groupByFieldType);
                     $groupBySelect['field'] = "ezcontentobject." . $groupByField;
                     break;
             }
             $outGroupBySelectText = ", " . $groupBySelect['select'];
             $outGroupByText = "GROUP BY " . $groupBySelect['group_field'];
         }
     }
 }