SQL AS:表别名和字段别名

 
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。

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];

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

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