题解 | #每份试卷每月作答数和截止当月的作答总数。#

1.先计算每月作答数count(*)

2.再计算截至当月sum() over()

select *,
sum(month_cnt) over(partition by exam_id order by start_month)
from (
select exam_id,
DATE_FORMAT(start_time,'%Y%m')start_month,
count(exam_id) month_cnt
from exam_record
group by DATE_FORMAT(start_time,'%Y%m'),exam_id
)tt