select qd.difficult_level,count(if(qpd.result='right',1,null))/count(qpd.question_id) as correct_rate
from user_profile as up
join question_practice_detail as qpd
    on up.device_id=qpd.device_id
join question_detail as qd
    on qd.question_id=qpd.question_id

where up.university='浙江大学'
group by qd.difficult_level
order by correct_rate asc