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
from
  (
    select
      exam_id,
      date_format(start_time, '%Y%m') as start_month,
      count(start_time) month_cnt
    from
      exam_record
    group by
      exam_id,
      date_format(start_time, '%Y%m')
  ) t

高赞优化：

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