题解 | #每份试卷每月作答数和截止当月的作答总数。#

select a.*,
sum(a.month_cnt) over(partition by a.exam_id order by a.start_month)

from
(
select
er.exam_id,
date_format(er.start_time,'%Y%m') as start_month,
count(1) as month_cnt
from exam_record er
group by er.exam_id,date_format(er.start_time,'%Y%m')
) a