# 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;