SQL Aliases 别名


SQL Aliases 别名

通过使用 SQL,可以为表名称或列名称指定别名(Alias)。

SQL 别名(Aliases)

  • SQL 别名用于为表或表中的列提供临时名称。

  • SQL 别名通常用于使列名更具可读性。

  • SQL 一个别名只存在于查询期间。

列的 SQL Alias 语法

SELECT _column_name_ AS _alias_name_  
 FROM _table_name;_

表的 SQL Alias 语法

SELECT column_name(s)
FROM table_name AS alias_name;

演示数据库

在本教程中,我们将使用著名的Northwind示例数据库。

以下是"Customers" 表中的数据:

CustomerID CustomerName ContactName Address City PostalCode Country
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

下面是选自 "Orders" 表的数据:

OrderID CustomerID EmployeeID OrderDate ShipperID
10354 58 8 1996-11-14 3
10355 4 6 1996-11-15 1
10356 86 6 1996-11-18 2

列的 Alias 实例

以下SQL语句创建两个别名,一个用于CustomerID列,另一个用于CustomerName列:

实例

SELECT CustomerID as ID, CustomerName AS Customer  
FROM Customers;

以下SQL语句创建两个别名,一个用于CustomerName列,一个用于ContactName列。 注: 如果别名包含空格,则需要双引号或方括号:

实例

SELECT CustomerName AS Customer, ContactName AS [Contact Person]  
FROM Customers;

以下SQL语句创建一个名为“Address”的别名,它包含四列(Address,PostalCode,City and Country):

SELECT CustomerName, Address + ', ' + PostalCode + ' ' + City + ', ' + Country AS Address
FROM Customers;

注意: 要使上面的SQL语句在MySQL中工作,请使用以下命令:

SELECT CustomerName, CONCAT(Address,', ',PostalCode,', ',City,', ',Country) AS Address
FROM Customers;

表的 Alias 实例

以下SQL语句选择CustomerID = 4(“围绕角”)的所有订单。我们使用“Customers”和“Orders”表,给它们分别为“c”和“o”的表别名(这里我们使用别名来使SQL更短):

实例

SELECT o.OrderID, o.OrderDate, c.CustomerName  
FROM Customers AS c, Orders AS o  
WHERE c.CustomerName="Around the Horn" AND c.CustomerID=o.CustomerID;

以下SQL语句与上述相同,但没有别名:

实例

SELECT Orders.OrderID, Orders.OrderDate, Customers.CustomerName  
FROM Customers, Orders  
WHERE Customers.CustomerName="Around the Horn" AND
Customers.CustomerID=Orders.CustomerID;

在下列情况下使用别名是有用的:

  • 查询涉及多个表

  • 用于查询函数

  • 需要把两个或更多的列放在一起

  • 列名长或可读性差

语法:

表别名 的基本语法如下:

SELECT column1, column2....
FROM table_name AS alias_name
WHERE [condition];

列别名 的基本语法如下:

SELECT column_name AS alias_name
FROM table_name
WHERE [condition];

示例:

考虑下面两个数据表,(a)CUSTOMERS 表,如下:

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

(b)另一个是 ORDERS 表,如下所示:

+-----+---------------------+-------------+--------+
|OID  | DATE                | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 |           3 |   3000 |
| 100 | 2009-10-08 00:00:00 |           3 |   1500 |
| 101 | 2009-11-20 00:00:00 |           2 |   1560 |
| 103 | 2008-05-20 00:00:00 |           4 |   2060 |
+-----+---------------------+-------------+--------+

下面是 表别名 的用法:

SQL> SELECT C.ID, C.NAME, C.AGE, O.AMOUNT
        FROM CUSTOMERS AS C, ORDERS AS O
        WHERE  C.ID = O.CUSTOMER_ID;

上面语句的运行结果如下所示:

+----+----------+-----+--------+
| ID | NAME     | AGE | AMOUNT |
+----+----------+-----+--------+
|  3 | kaushik  |  23 |   3000 |
|  3 | kaushik  |  23 |   1500 |
|  2 | Khilan   |  25 |   1560 |
|  4 | Chaitali |  25 |   2060 |
+----+----------+-----+--------+

下面是 列别名 的用法:

SQL> SELECT  ID AS CUSTOMER_ID, NAME AS CUSTOMER_NAME
     FROM CUSTOMERS
     WHERE SALARY IS NOT NULL;

其运行结果如下所示:

+-------------+---------------+
| CUSTOMER_ID | CUSTOMER_NAME |
+-------------+---------------+
|           1 | Ramesh        |
|           2 | Khilan        |
|           3 | kaushik       |
|           4 | Chaitali      |
|           5 | Hardik        |
|           6 | Komal         |
|           7 | Muffy         |
+-------------+---------------+