select u.university, 
round(count(q.question_id)/count(distinct q.device_id),4) as avg_answer_cnt
from user_profile u 
join question_practice_detail q
on u.device_id = q.device_id
group by u.university 
order by u.university

按照大学分组之后，进行两表连接，然后加count函数 就做出来