SQL34 每份试卷每月作答数和截止当月的作答总数

题目主要信息：

• 输出每份试卷每月作答数和截止当月的作答总数

问题拆分：

• 求每份试卷每个月的作答数，要以exam_id，开始作答时间的月份进行分组。group by exam_id, month(start_time) 知识点：group by
• 直接获取试卷ID、作答月份。
• 对每组开始作答时间计数得到每份试卷每个月的作答数。知识点：count()
• 以试卷分组，月份递增获取每份试卷每个月的累计和。sum(count(start_time)) over(partition by exam_id order by month(start_time)) as cum_exam_cnt 知识点：sum()over()、count()、partition by、order by

代码：

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 month(start_time)) as cum_exam_cnt
from exam_record
group by exam_id, month(start_time)