select university , difficult_level ,round(count(c.question_id)/count(distinct a.device_id),4)
from user_profile AS a join question_practice_detail AS b on a.device_id=b.device_id 
join question_detail AS c on b.question_id=c.question_id 
group by university , difficult_level

本次考察的是对多表联合的理解