题解 | #每份试卷每月作答数和截止当月的作答总数。#
每份试卷每月作答数和截止当月的作答总数。
http://www.nowcoder.com/practice/5f1cbe74c682485aa73e4c2b30f04a62
select *, sum(ct)over(partition by exam_id order by dt) from (select exam_id,date_format(start_time,'%Y%m') dt,count(start_time) ct from exam_record group by exam_id,dt) a