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极客学院团队出品 · 更新于 2018-07-20 20:00:41

SQL 别名

我们可以使用别名(Alias)来对数据表或者列进行临时命名。

使用别名意味着要用特定的 SQL 语句对表进行重命名。重命名是临时的,数据库中表的实际名字并不会改变。

对于特定的 SQL 查询,需要使用列别名来对表中的列进行重命名。

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