SQL GROUP BY 语句


SQL GROUP BY 语句

Aggregate 函数常常需要添加 GROUP BY 语句。

GROUP BY语句通常与集合函数(COUNT,MAX,MIN,SUM,AVG)一起使用,以按一个或多个列对结果集进行分组。

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(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);

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 "Customers"表的数据:

CustomerID CustomerName ContactName Address City PostalCode Country
1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany
2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico
3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 México D.F. 05023 Mexico
4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK
5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S-958 22 Sweden

SQL GROUP BY示例

以下SQL语句列出了每个国家/地区的客户数量:

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country;

以下SQL语句列出每个国家的客户数量,从高到低排序:

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
ORDER BY COUNT(CustomerID) DESC;

演示数据库

以下是罗斯文示例数据库中“订单”表的一个选择:

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

并从“Shippers”表中选择:

ShipperID ShipperName
1 Speedy Express
2 United Package
3 Federal Shipping

GROUP BY使用JOIN示例

以下SQL语句列出了每个发货人发送的订单数量:

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