题解 | #每份试卷每月作答数和截止当月的作答总数。#

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