题解 | #统计每个学校各难度的用户平均刷题数#

select university,difficult_level,count(c.question_id)/count(distinct b.device_id)
from 
(select device_id,university from user_profile)a 
join
(select device_id,question_id from question_practice_detail)b
on a.device_id=b.device_id 
join 
question_detail c
on b.question_id=c.question_id
group by university, difficult_level

我写的时候遇到的问题：貌似join 和where将两个表连接不能同时出现？？？希望解答