题解 | 浙大不同难度题目的正确率

select
    qd.difficult_level,
    sum(if (result = "right", 1, 0)) / count(qpd.question_id) correct_rate
from
    user_profile up
    inner join question_practice_detail qpd on up.device_id = qpd.device_id
    inner join question_detail qd on qpd.question_id = qd.question_id
where
    university = "浙江大学"
group by
    qd.difficult_level
order by
    correct_rate

先把简单的写好再细化一下，我开始用的left join 有一个用例没有通过，多输出了一行null，有大佬给我解释一下为什么嘛