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 exam_id,start_month,month_cnt,sum(month_cnt)over(partition by exam_id order by start_month) from
(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) a
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