select case when start_time like '2021-01%' then '202101' when start_time like '2021-02%' then '202102' when start_time like '2021-03%' then '202103' when start_time like '2021-04%' then '202104' when start_time like '2021-05%' then '202105' when start_time like '2021-06%' then '202106' when start_time like '2021-07%' then '202107' when start_time like '2021-08%' then '202108' when start_time like '2021-09%' then '202109' when start_time like '2021-10%' then '202110' when start_time like '2021-11%' then '202111' when start_time like '2021-12%' then '202112' else end as act_month, count(uid)/count(act_month),count(uid) from exam_record group by month; 大佬我这个为啥报错