select t3.difficult_level, sum(if(t2.result = 'right',1,0))/count(1) as correct_rate from (select device_id from user_profile where university = '浙江大学')t1 join question_practice_detail t2 on t1.device_id = t2.device_id join question_detail t3 on t2.question_id = t3.question_id group by t3.difficult_level order by correct_rate