select difficult_level,  sum(if(result='right',1,0))/ count(q.question_id)  correct_rate from user_profile u 
inner join question_practice_detail q
on u.device_id = q.device_id 
inner join question_detail q1
on q.question_id = q1.question_id
where  university = '浙江大学'
group by difficult_level
order by correct_rate