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