题解 | #每份试卷每月作答数和截止当月的作答总数。#

select
    *,
    SUM(t.month_cnt) OVER (
        partition by t.exam_id
        order by
            t.exam_id asc,t.start_month asc
    ) cum_exam_cnt
from
    (
        select
            exam_id,
            date_format (start_time, '%Y%m') start_month,
            count(start_time) month_cnt
        from
            exam_record
        group by
            exam_id,
            start_month
    ) as t