题解 | #浙大不同难度题目的正确率#
浙大不同难度题目的正确率
http://www.nowcoder.com/practice/d8a4f7b1ded04948b5435a45f03ead8c
select c.difficult_level, sum(case when b.question_id is null then 0 when b.result='wrong' then 0 else 1 end)/sum(case when b.question_id is null then 0 else 1 end) as correct_rate from user_profile a left join question_practice_detail b on(a.device_id=b.device_id and a.university='浙江大学') left join question_detail c on(b.question_id=c.question_id) group by c.difficult_level having c.difficult_level is not null order by correct_rate