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：
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：
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;
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