题解 | #统计每个用户的平均刷题数#

select 
	university,difficult_level,round(count(qpd.question_id) / count(distinct qpd.device_id),4)
from 
	user_profile as up,question_practice_detail as qpd,question_detail as qd
where 
	up.device_id=qpd.device_id and qpd.question_id=qd.question_id and university='山东大学'
group by 
	qd.difficult_level

我们知道，sql语句的执行顺序是：

from

where

group by

having

select

order by

limit

根据题意：'运营想要查看参加了答题的山东大学的用户在不同难度下的平均答题题目数'

首先是个多表联查的问题，然后不同难度则需要分组