题解 | #每份试卷每月作答数和截止当月的作答总数。#

select
    *,
    sum(month_cnt) over (
        partition by
            exam_id
        order by
            start_month
    )
from
    (
        select
            exam_id,
            date_format (start_time, "%Y%m") as start_month,
            count(1) as month_cnt
        from
            exam_record
        group by
            exam_id,
            date_format (start_time, "%Y%m")
    ) a