题解 | #每份试卷每月作答数和截止当月的作答总数。#

select exam_id,
    dt, cnt_m,
    sum(cnt_m) over ( partition by exam_id order by dt) as cum_cnt    # 按exam_id分组，按时间排序后计算累计值
from
    (select exam_id, dt,cnt_m
        from
            (select exam_id,date_format (start_time, '%Y%m') dt,
                    count(start_time) as cnt_m
                from exam_record
                group by
                    exam_id,
                    date_format (start_time, '%Y%m')
            ) t
    ) t1

主要是sum()over()函数的应用：向上累计相加，第n行的值为 n+n-1+n-2+......+1

第一步：统计每个月的作答记录数

count(start_time)

第二步：计算累计作答数

sum(cnt_m) over ( partition by exam_id order by dt) 按试卷号分组后，按时间排序，向上累计相加值