题解 | 浙大不同难度题目的正确率

select
    qd.difficult_level,
    sum(case when result = 'right' then 1 else 0 end) / count(qpd.result) as correct_rate
from 
    user_profile u
left join question_practice_detail qpd on u.device_id = qpd.device_id
left join question_detail qd on qpd.question_id = qd.question_id
where university = '浙江大学' and qd.difficult_level is not NULL
group by qd.difficult_level
order by correct_rate