题解 | #每份试卷每月作答数和截止当月的作答总数。#

select exam_id,start_month,month_cnt,sum(month_cnt)over(partition by exam_id order by start_month) cum_exam_cnt
from(
select exam_id
,date_format(start_time,'%Y%m') start_month
,count(date_format(start_time,'%Y%m')) month_cnt from
exam_record
group by exam_id,date_format(start_time,'%Y%m')
) a;