select
    difficult_level,
    round(sum(if(qpd.result = 'right', 1, 0)) / count(qpd.result), 4) correct_rate
from
user_profile up 
inner join
question_practice_detail qpd on
up.device_id = qpd.device_id
inner join
question_detail qd on
qpd.question_id = qd.question_id
where university = '浙江大学'
group by
qd.difficult_level
order by 
correct_rate