本题,重点考察计算模式。
有一点很重要-->本题,order by不能应用到本题,会报错。
	order by 仅能用到 where后边,否则会报错
select 
    b.university,
    round(count(a.device_id)/count(distinct(b.device_id)),4)  avg_answer_cnt
from 
    question_practice_detail a
inner join
    user_profile b
on
    a.device_id = b.device_id  
group by
    b.university;