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SQL GROUP BY 详解及简单实例

来源:http://www.dnamique.com 作者:计算机网络 人气:152 发布时间:2020-04-14
摘要: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_nameWHERE column_name operator valueGROUP BY column_name; 演

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_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 (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 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:

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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