select difficult_level,
       avg(if(qpd.result="right",1,0)) as correct_rate #平均函数和if嵌套
from user_profile as u
join question_practice_detail as qpd
on u.device_id=qpd.device_id
join question_detail as qd
on qpd.question_id = qd.question_id

where u.university='浙江大学'
group by difficult_level
order by correct_rate