题解 | #每份试卷每月作答数和截止当月的作答总数。#

select 
exam_id,
date_format(start_time,"%Y%m") as start_month,
count(start_time) as month_cnt,
sum(count(start_time)) over (partition by exam_id order by date_format(start_time,"%Y%m")) as cum_exam_cnt 
from 
exam_record  
group by exam_id,start_month

# 考核知识点：

# 聚合窗口函数的使用，sum() over (patition by 字段 order by 字段 )

# patition by 是分组依据 order by 是排序依据 ，sum(count(start_time))是总和统计start_time的次数和,其中容易出错的地方是over()里面的排序依据，不可以直接写 order by start_month 会报错，必须写成date_format(start_time,"%Y%m")

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