题解 | #统计每个学校的答过题的用户的平均答题数#

select
    university,
    count(question_practice_detail.question_id) / count(distinct user_profile.device_id) as 'avg_answer_cnt'
from 
    user_profile
inner join
    question_practice_detail
on 
    user_profile.device_id = question_practice_detail.device_id
group by    
    university