题解 | 统计每个用户的平均刷题数

select
    university,
    difficult_level,
    round(count(result) / count(distinct up.device_id), 4) avg_answer_cnt
from
    question_practice_detail qpd
    join user_profile up on qpd.device_id = up.device_id
    join question_detail qd on qpd.question_id = qd.question_id
where
    university = '山东大学'
group by
    university,
    difficult_level