题解 | 统计每个用户的平均刷题数

# 统计每个用户的平均刷题数目
select
    up.university,
    qd.difficult_level,
    count(qpd.question_id)/count(distinct qpd.device_id
) as avg_answer_cnt
from
    user_profile as up,
    question_practice_detail as qpd ,
    question_detail as qd 
where up.university="山东大学" and
    up.device_id=qpd.device_id and 
    qpd.question_id=qd.question_id
group by qd.difficult_level;