SQL GROUP BY 詳解及復雜實例

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 GROUP BY 語句用於結合 Aggregate 函數，依據一個或多個列對後果集停止分組。

SQL GROUP BY 語法

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

 演示數據庫

在本教程中，我們將運用眾所周知的 Northwind 樣本數據庫。

上面是選自 "Orders" 表的數據：

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

選自 "Shippers" 表的數據：

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

選自 "Employees" 表的數據：

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 實例

如今我們想要查找每個送貨員配送的訂雙數目。

上面的 SQL 語句按送貨員停止訂單分類統計：

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

GROUP BY 一個以上的列

我們也可以對一個以上的列使用 GROUP BY 語句，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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