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_id) / count(distinct qpd.device_id) as avg_answer_cnt
from
    question_practice_detail as qpd
    join user_profile as u on qpd.device_id = u.device_id
group by
    university;