SQL GROUP BY 详解及简单实例

 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

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1234	SELECT column_name, aggregate_function(column_name)FROM table_nameWHERE column_name operator valueGROUP 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	[phone]
2	United Package	[phone]
3	Federal Shipping	[phone]

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：

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1234	SELECT Shippers.ShipperName,COUNT(Orders.OrderID) AS NumberOfOrders FROM OrdersLEFT JOIN ShippersON Orders.ShipperID=Shippers.ShipperIDGROUP 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：

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SELECT Shippers.ShipperName, Employees.LastName,COUNT(Orders.OrderID) AS NumberOfOrdersFROM ((OrdersINNER JOIN ShippersON Orders.ShipperID=Shippers.ShipperID)INNER JOIN EmployeesON Orders.EmployeeID=Employees.EmployeeID)GROUP BY ShipperName,LastName;

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