final public function groupBy($fields) {
      if (func_num_args()>0) {
         if (func_num_args()==1) {
            if (!is_array($fields)) {
               $fields = (array) $fields;
            }
         }else{
            $fields = func_get_args();
         }

         $this->groupBy = " GROUP BY ".implode(',',$fields);

         $this->find_first_data = $this->find_save_data = $this->find_delete_data = FALSE;
         
         $this->get_instance();
         return $this;
      }
      return null;
   }