题解 | #每份试卷每月作答数和截止当月的作答总数。#

select DISTINCT 
    exam_id,
    DATE_FORMAT(start_time,'%Y%m') as start_month,
    count(start_time) over(partition by exam_id,date_format(start_time,'%Y%m')) as month_cnt,
        count(start_time) over(partition by exam_id order by date_format(start_time,'%Y%m')) as cum_exam_cnt
from exam_record
order by exam_id,start_month