题解 | #每份试卷每月作答数和截止当月的作答总数。#

select exam_id,
        start_month,
        month_cnt,
        sum(month_cnt)over(partition by exam_id
                          order by start_month) cum_exam_cnt
        # 窗口函数，按exam_id分组，按start_month排序后求和至当前月之前的所有作答数
from
    (select exam_id,
     date_format(start_time,'%Y%m') start_month,
     count(start_time) month_cnt
     from exam_record
     group by date_format(start_time,'%Y%m'),exam_id   
    ) temp