题解 | #每份试卷每月作答数和截止当月的作答总数。#

select
    distinct    
    exam_id,
    date_format (start_time, '%Y%m'),
    count(start_time) over (partition by date_format        (start_time, '%Y%m'), exam_id ) as month_cnt,
    count(start_time) over (partition by exam_id order by date_format(start_time, '%Y%m'))
from
    exam_record
    order by exam_id asc

本题难度在于聚合函数也可以用over() partition by 函数去分区，然后使用orderby去排序(本质也是分区)