select qd.difficult_level,
    round(sum(qp.result='right')/count(qd.question_id),4) as correct_rate
from user_profile u
    join question_practice_detail qp on u.device_id=qp.device_id and university='浙江大学'
    join question_detail qd on qp.question_id=qd.question_id
group by qd.difficult_level
order by correct_rate