MySQL | INNER JOIN ON 多个表

如果需要连接多个表的话可以这样写：

# 参加了答题的山东大学的用户在不同难度下的平均答题题目数
select 
    "山东大学" as university,
    difficult_level,
    count(qpd.question_id) / count(distinct qpd.device_id) as avg_answer_cnt
from 
    question_practice_detail as qpd
inner join# 存在数据才显示
    user_profile as up
on
    up.device_id=qpd.device_id and up.university="山东大学"
inner join 
    question_detail as qd
on
    qd.question_id=qpd.question_id
group by
    difficult_level