思路：①先分组聚合算出每月答题数目；②用sum窗口函数算累加数
select *, sum(c)over(partition by exam_id order by start_month)
from(
select exam_id, date_format(start_time,'%Y%m') start_month, count(start_time) c
from exam_record
group by exam_id, date_format(start_time,'%Y%m')) as tb1