select
    difficult_level,
    round(sum(
            if(qp.result='right',1,0)
    )/count(qp.question_id),4)
    as 
        correct_rate
from
    question_detail as qt
join
    question_practice_detail as qp 
    on
    qt.question_id = qp.question_id
join
    user_profile as ue
    on
    ue.device_id = qp.device_id
where
    ue.university = '浙江大学'
group by
    qt.difficult_level
order by
    correct_rate
;