题解 | #每份试卷每月作答数和截止当月的作答总数。#

SELECT exam_id, start_month, month_cnt, 
       SUM(month_cnt) OVER(PARTITION BY exam_id ORDER BY start_month) AS cum_exam_cnt
FROM(
    SELECT exam_id, DATE_FORMAT(start_time,"%Y%m") AS start_month, COUNT(DISTINCT id) AS month_cnt
    FROM exam_record
    GROUP BY exam_id, DATE_FORMAT(start_time,"%Y%m")
) AS t;

    

① 先得到不同exam_id和作答日期的month_cnt表

② 基于①得到的表，得到累加的表格。