题解 | 每份试卷每月作答数和截止当月的作答总数。

select
    exam_id,
    date_format (start_time, '%Y%m') start_month,
    count(start_time) month_cnt,
    sum(count(start_time)) over (
        partition by
            exam_id
        order by
            date_format (start_time, '%Y%m') rows between unbounded preceding
            and current row
    ) cum_exam_cnt
from
    test.exam_record
group by
    exam_id,
    date_format (start_time, '%Y%m')