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

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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