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;
