40. SQL--as：表别名和字段别名

1. 前言

sql as 关键字用于为表或字段起一个临时的别名。别名是临时的，它仅在当前 sql 语句中奏效，数据库中的实际表名和字段名不会更改。

select 命令的结果集中将显示别名，而不是原始名。

通常在下列情况中使用别名：

• 有两个名字重复的表，需要为其中一个表起一个别名加以区分，比如 self join。
• 两个表中有重复的字段名，起别名加以区分。
• 表名/字段名较长，或者可读性差。

2. 语法

表别名的基本语法如下：

select column1, column2....
from table_name as alias_name
where [condition];

字段别名的基本语法如下：

select column_name as alias_name
from table_name
where [condition];

3. 示例

现在有以下两个表，分别是客户表和订单表。

表1：CUSTOMERS 表

+----+----------+-----+-----------+----------+
| 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 |
+----+----------+-----+-----------+----------+

表2：ORDERS 表

+-----+---------------------+-------------+--------+
|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 |
+-----+---------------------+-------------+--------+

1) 为表起一个别名，代码如下：

sql> select c.id, c.name, c.age, o.amount
     from customers as c, orders as o
     where  c.id = o.customer_id;

执行结果：

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

2) 为字段起一个别名，代码如下：

sql> select  id as customer_id, name as customer_name
     from customers
     where salary is not null;

执行结果：

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