SQL 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。

## 语法

```sql
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;
```

## 实例

### 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.... |

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：

```sql
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：

```sql
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;
```