题解 | #每份试卷每月作答数和截止当月的作答总数。#

select *,sum(month_cnt) over(partition by exam_id order by start_month)
from
(select distinct exam_id, date_format(start_time,'%Y%m') start_month,
count(start_time) over(partition by exam_id,date_format(start_time,'%Y%m')) month_cnt
from exam_record) t1

sum(month_cnt) over(partition by exam_id order by start_month) ：根据月份排序可以计算出开始到当前月份的累计和（不是很理解求助）