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。
https://www.nowcoder.com/practice/5f1cbe74c682485aa73e4c2b30f04a62
SUM() OVER(ORDER BY ) 可以计算累积频数
SELECT exam_id, start_month, month_cnt, SUM(month_cnt) OVER(PARTITION BY exam_id ORDER BY exam_id,start_month) AS cum_exam_cnt FROM (SELECT exam_id, DATE_FORMAT(start_time,'%Y%m') AS start_month, COUNT(start_time) AS month_cnt FROM exam_record WHERE start_time IS NOT NULL GROUP BY exam_id,start_month)t1 # SUM() OVER(ORDER BY ) 可以计算累积频数