SQL GROUP BY 语句
SQL GROUP BY 语句
GROUP BY语句将具有相同值的行分组到汇总行中,例如“查找每个国家的客户数”。
GROUP BY语句通常与聚合函数(COUNT,MAX,MIN,SUM,AVG)一起使用,将结果集分组为一列或多列。
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
以下是Northwind示例数据库中“Customers”表的选择:
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 Constituciуn 2222 | Mйxico D.F. | 05021 | Mexico |
3 | Antonio Moreno Taquerнa | Antonio Moreno | Mataderos 2312 | Mйxico D.F. | 05023 | Mexico |
4 | Around the Horn | Thomas Hardy | 120 Hanover Sq. | London | WA1 1DP | UK |
5 | Berglunds snabbkцp | Christina Berglund | Berguvsvдgen 8 | Luleе | S-958 22 | Sweden |
SQL GROUP BY 实例
以下SQL语句列出了每个国家的客户数量:
以下SQL语句列出了每个国家的客户数量,从高到低排序:
实例
SELECT COUNT(CustomerID), Country FROM Customers GROUP BY Country ORDER BY COUNT(CustomerID) DESC;
演示数据库
以下是Northwind实例数据库中“Orders”表查询:
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”表中查询:
ShipperID | ShipperName |
---|---|
1 | Speedy Express |
2 | United Package |
3 | Federal Shipping |
GROUP BY 和 JOIN 实例
以下SQL语句列出了每个出货单发送的订单数:
实例
SELECT Shippers.ShipperName, COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders LEFT JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID GROUP BY ShipperName;