结果筛选——having

where 和 having 的区别

• where 是对原表进行筛选，是在结果返回之前起作用的，不能使用聚合函数

• having 是对查询结果（结果集 / 伪表）进行过滤，可以使用聚合函数

Example

SELECT
	address,
	avg( age ) AS 'avg_age' 
FROM
	manager 
HAVING
	age >= 24;

SELECT
	classid,
	count(*) AS 'num_of_student' 
FROM
	student 
GROUP BY
	classid;
HAVING
	count <=10;