select qq.difficult_level,
count(if (q.result='right',1,null))/count(q.question_id) as correct_rate

from user_profile u
 join question_practice_detail q
on u.device_id = q.device_id
 join question_detail qq
on q.question_id = qq.question_id
where u.university ='浙江大学'
group by qq.difficult_level
order by correct_rate asc

算了半天发现跟标答不一样，结果发现看成了复旦大学 啊啊啊啊审题！