题解 | #每份试卷每月作答数和截止当月的作答总数。#

#思路：①先计算每个试卷在每个月的作答次数；②再用窗口函数针对每个试卷进行逐月累加即可

select exam_id, mt, a, sum(a)over(partition by exam_id order by mt)
from(
select exam_id, date_format(start_time,'%Y%m') mt, count(exam_id) a
from exam_record
group by exam_id, mt) as tb1