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