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 语句用于结合 Aggregate 函数,根据一个或多个列对结果集进行分组。
SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);
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 |
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;
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 |
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;