select difficult_level,
sum(if(qpd.result = 'right',1,0))/count(result) correct_rate
from user_profile as up
inner join question_practice_detail as qpd on up.device_id = qpd.device_id
inner join question_detail as qd on qpd.question_id = qd.question_id
where university = '浙江大学'
group by difficult_level
order by correct_rate