# SQL GROUP BY 语句

Aggregate 函数常常需要添加 GROUP BY 语句。

## GROUP BY 语句

GROUP BY 语句用于结合 Aggregate 函数，根据一个或多个列对结果集进行分组。

### SQL GROUP BY 语法

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

## 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 "Shippers" 表的数据：

ShipperID | ShipperName | Phone
--- | --- | ---
1 | Speedy Express | (503) 555-9831
2 | United Package | (503) 555-3199
3 | Federal Shipping | (503) 555-9931

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 GROUP BY 实例

现在我们想要查找每个送货员配送的订单数目。

下面的 SQL 语句按送货员进行订单分类统计：

## 实例

     SELECT Shippers.ShipperName,COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders
    LEFT JOIN Shippers
    ON Orders.ShipperID=Shippers.ShipperID
     GROUP BY ShipperName;

## GROUP BY 一个以上的列

我们也可以对一个以上的列应用 GROUP BY 语句，如下所示：

## 实例

     SELECT Shippers.ShipperName, Employees.LastName,
     COUNT(Orders.OrderID) AS NumberOfOrders
     FROM ((Orders
    INNER JOIN Shippers
    ON Orders.ShipperID=Shippers.ShipperID)
    INNER JOIN Employees
    ON Orders.EmployeeID=Employees.EmployeeID)
    GROUP BY ShipperName,LastName;