select
  difficult_level,
    //正确率：正确数/总题数
  sum(if(qpd.result = 'right', 1, 0)) / count(qpd.question_id) as correct_rate
from
  question_practice_detail qpd
  inner join question_detail qd on qd.question_id = qpd.question_id
  inner join user_profile up on up.device_id = qpd.device_id
where
  up.university = '浙江大学'
group by
  difficult_level //不同难度
order by
  correct_rate asc; //正确率升序排列