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
https://www.nowcoder.com/practice/d8a4f7b1ded04948b5435a45f03ead8c
select
d.difficult_level,
count(result = 'right' or null) / count(1) correct_rate
from
user_profile u,
question_practice_detail q,
question_detail d
where
u.device_id = q.device_id
and q.question_id = d.question_id
and university = '浙江大学'
group by
d.difficult_level
order by
correct_rate
# count(null)的值为0
## count('不为列名的任意值')相当于count(*)