题解 | #每份试卷每月作答数和截止当月的作答总数。#

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

计算累计值：用聚合窗口函数 sum(count(id)) over(partition by exam_id order by date_format(start_time,'%Y%m'))