function queryFilterEmpty(&$layout_def)
 {
     if ($this->reporter->db->dbType == 'mysql') {
         return parent::queryFilterEmpty($layout_def);
     } elseif ($this->reporter->db->dbType == 'mssql') {
         return '( ' . $this->_get_column_select($layout_def) . ' IS NULL OR DATALENGTH(' . $this->_get_column_select($layout_def) . ") = 0)\n";
     }
 }