题解 | 浙大不同难度题目的正确率

select qd.difficult_level,
count(case when qpd.result="right" then 1 end)
    /count(*) as correct_rate
from user_profile as up 
left join question_practice_detail as qpd on up.device_id = qpd.device_id
left join question_detail as qd on qpd.question_id = qd.question_id
where up.university = "浙江大学" and qd.difficult_level is not null
group by qd.difficult_level
order by correct_rate

三个表左连接即可，注意difficult_level有存在空的情况，需要排除none，最后输出按照correct_rate排序即可。