select city,
       max(sum_wait) max_wait_uv
from (select *,
              sum(tag) over(partition by city order by tm,tag desc) sum_wait
      from (select *
             from (select city,
                          a.uid,
			              event_time tm,
			              1 as tag
                   from tb_get_car_order a
                   left join tb_get_car_record b
                   on a.order_id = b.order_id
                   union
                   select city,
                          a.uid,
			              start_time tm,
			              -1 as tag
                   from tb_get_car_order a
                   left join tb_get_car_record b
                   on a.order_id = b.order_id
                   ) t
        where date_format(tm,'%Y-%m') = '2021-10'
              ) t
     ) t
group by city
order by max_wait_uv,city