题解 | 浙大不同难度题目的正确率

select qd.difficult_level,   sum((case when qpd.result='right' then 1 else 0 end))/ count(qpd.question_id) correct_rate
from question_practice_detail qpd left join user_profile u using(device_id)
     left join question_detail qd using(question_id)
where u.university='浙江大学'
group by qd.difficult_level
order by correct_rate