def _group_by(self):
        group_str = """
            GROUP BY    a.type_id,
                        a.vehicle_id,
                        a.driver_id,
                        a.co_driver_id,
                        a.date_start,
                        a.date_end,
                        a.real_date_depart,
                        a.real_date_arrive,
                        a.start_location_id,
                        a.end_location_id,
                        a.state
        """
        return group_str