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 snabbk"op |
Christina Berglund |
Berguvsv"agen 8 |
Lulea |
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;