Having和Where+subqueries的转换

Having表示对上述结果进行过滤，所以比Where更加灵活。Where之后不能跟聚合函数，因为where表示对每一行进行条件限制，此时聚合函数的值还没有求出，但是Having可以跟聚合函数，因为Having表示对结果进行过滤。

例子：求平均薪资大约2000的部门及部门薪资
参数：

		employee表，包含所有信息
		department 部门，string类型
        salary 每个人的薪水，int类型

下面这个两个答案是错的：（where之后不能用聚合函数的信息）
答案1：

SELECT department,
	   avg(salary) AS sal
FROM   employee
GROUP BY department
WHERE sal>2000   #Where在group by之前生效，所以就是说where后面不能写聚合函数

答案2：

SELECT department,
	   avg(salary) AS sal
FROM   employee
GROUP BY department
WHERE avg(salary)>2000   #Where在group by之前生效，所以就是说where后面不能写聚合函数

正确答案：

1. 用Having

SELECT department,
	   avg(salary) AS sal
FROM   employee
GROUP BY department
HAVING sal>2000   

2. 用Where+subqueries

SELECT department,
	   sal
FROM   (SELECT department,
	   		   avg(salary) AS sal
	   FROM   employee
	   GROUP BY department)
WHERE sal>2000