题解 | 浙大不同难度题目的正确率

select
    qd.difficult_level,
    sum(if(qpd.result = 'right', 1, 0)) / count(qpd.question_id) correct_rate
	# if函数别忘了要指定两个返回值
from
    (
        select
            device_id,
            university
        from
            user_profile
        where
            university = '浙江大学'
    ) as x # 子查询并不是必须的，也可以直接连接
    inner join question_practice_detail qpd on x.device_id = qpd.device_id
    inner join question_detail qd on qpd.question_id = qd.question_id
group by
    1
order by
    2 ASC