问题分析

1、每份试卷每月作答数——GROUP BY + COUNT() 2、每份试卷截止当月的作答总数——SUM()OVER()

答案参考

SELECT
    exam_id,date_format(start_time,'%Y%m')start_month
    ,count(start_time)mouth_cnt
    ,sum(count(start_time))over(partition by exam_id order by date_format(start_time,'%Y%m'))cum_exam_cnt
FROM
    exam_record
GROUP BY
    exam_id,date_format(start_time,'%Y%m');