SQL GROUP BY 语句
Aggregate 函数常常需要添加 GROUP BY 语句。
GROUP BY 语句
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：
实例
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：
实例
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;
