题解 | #SQL35 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 qd.difficult_level,
# 这里分子用 sum，求该列上所有值的和；分母用 count，求有多少个非空行
sum(if(qpd.result = 'right', 1, 0)) / count(qpd.question_id) correct_rate
# AVG(IF(result='right',1,0)) AS correct_rate
from user_profile u
join question_practice_detail qpd
on u.device_id = qpd.device_id
join question_detail qd
on qpd.question_id = qd.question_id
where u.university = '浙江大学'
group by difficult_level
order by correct_rate;
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