题解 | #分组过滤练习题#

SELECT university,
round(avg(question_cnt),3) as avg_question_cnt,
round(avg(answer_cnt),3) as avg_answer_cnt
from user_profile
group by university
having avg_question_cnt<5 or avg_answer_cnt<20

这里不适用where的原因是因为：where先于avg()/sum()执行，那时候还没有计算出平均值，就会出错。

报错信息：

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE avg_question_cnt<5 or avg_answer_cnt<20' at line 6