题解 | #每份试卷每月作答数和截止当月的作答总数。#

select *,#第二步：试卷内按日期排序累计试卷作答记录
sum(month_cnt) over(partition by exam_id order by start_month
	rows between unbounded preceding and current row) cum_exam_cnt
from(#第一步：统计每个试卷各月的作答数
	select exam_id,date_format(start_time,"%Y%m") start_month,
	count(exam_id) month_cnt
	from exam_record
	group by exam_id,start_month
	) as 第一步的表;