SQL HAVING 子句

HAVING 子句

在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与 Aggregate 函数一起使用。

SQL HAVING 语法

 SELECT column_name, aggregate_function(column_name)
 FROM table_name
 WHERE column_name operator value
 GROUP BY column_name
 HAVING aggregate_function(column_name) operator value;

演示数据库

在本教程中，我们将使用众所周知的 Northwind 样本数据库。

下面是选自 "Orders" 表的数据：

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10248	90	5	1996-07-04	3
10249	81	6	1996-07-05	1
10250	34	4	1996-07-08	2

选自 "Employees" 表的数据：

EmployeeID	LastName	FirstName	BirthDate	Photo	Notes
1	Davolio	Nancy	1968-12-08	EmpID1.pic	Education includes a BA....
2	Fuller	Andrew	1952-02-19	EmpID2.pic	Andrew received his BTS....
3	Leverling	Janet	1963-08-30	EmpID3.pic	Janet has a BS degree....

SQL HAVING 实例

现在我们想要查找订单总数少于 2000 的客户。

我们使用下面的 SQL 语句：

下面的 SQL 语句查找注册超过 10 个订单的员工：

实例

 SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM (Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID)
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 10;

现在我们想哟啊查找员工 "Davolio" 或 "Fuller" 是否拥有超过 25 个订单。

我们在 SQL 语句中增加一个普通的 WHERE 子句：

实例

 SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID
 WHERE LastName='Davolio' OR LastName='Fuller'
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 25;