题解 | #浙大不同难度题目的正确率#

## 先查出浙江大学学生的答题情况，然后与难度表链接。
select difficult_level,sum(if(result="right",1,0))/count(*) as correct_rate 
from (select * from question_practice_detail
where
    device_id in (
        select
            device_id
        from
            user_profile
        where
            university = "浙江大学"
    ))cc left join question_detail using(question_id)
group by difficult_level
order by correct_rate