题解 | #统计每个用户的平均刷题数#

错误原因：
好几次都没有看清楚表的结构，导致浪费大量时间找出错误

select
    university,
    t3.difficult_level,
    count(t2.question_id)/count(distinct t2.device_id) as avg_answer_cnt
from
    user_profile t1,
    question_practice_detail t2,
    question_detail t3
where
    t1.device_id = t2.device_id
    and t2.question_id = t3.question_id
    and university = '山东大学'
group by t3.difficult_level;