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 question_practice_detail.device_id) as avg_answer_cnt
from question_practice_detail
inner join user_profile
on user_profile.device_id = question_practice_detail.device_id
group by university