我们可以用 SQL 重命名一张表或者一个字段的名称，这个名称就叫着该表或该字段的别名。
创建别名是为了让表名或列名的可读性更强。
SQL 中 使用 AS 来创建别名。
语法
表的别名语法:
SELECT column1, column2....FROM table_name AS alias_nameWHERE [condition];
列的别名语法:
SELECT column_name AS alias_nameFROM table_nameWHERE [condition];
实例
创建 COMPANY 表（下载 COMPANY SQL 文件 ），数据内容如下：
runoobdb# select * from COMPANY; id | name | age | address | salary----+-------+-----+-----------+-------- 1 | Paul | 32 | California| 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 6 | Kim | 22 | South-Hall| 45000 7 | James | 24 | Houston | 10000(7 rows)
创建 DEPARTMENT 表（下载 COMPANY SQL 文件 ），数据内容如下：
runoobdb=# SELECT * from DEPARTMENT; id | dept | emp_id----+-------------+-------- 1 | IT Billing | 1 2 | Engineering | 2 3 | Finance | 7 4 | Engineering | 3 5 | Finance | 4 6 | Engineering | 5 7 | Finance | 6(7 rows)
下面我们分别用 C 和 D 表示 COMPANY 表和 DEPAERMENT 表的别名：
runoobdb=# SELECT C.ID, C.NAME, C.AGE, D.DEPT FROM COMPANY AS C, DEPARTMENT AS D WHERE C.ID = D.EMP_ID;
得到结果如下：
id | name | age | dept----+-------+-----+------------ 1 | Paul | 32 | IT Billing 2 | Allen | 25 | Engineering 7 | James | 24 | Finance 3 | Teddy | 23 | Engineering 4 | Mark | 25 | Finance 5 | David | 27 | Engineering 6 | Kim | 22 | Finance(7 rows)
下面，我们用 COMPANY_ID 表示 ID 列，COMPANY_NAME 表示 NAME 列，来展示列别名的用法：
runoobdb=# SELECT C.ID AS COMPANY_ID, C.NAME AS COMPANY_NAME, C.AGE, D.DEPT FROM COMPANY AS C, DEPARTMENT AS D WHERE C.ID = D.EMP_ID;
得到结果如下：
company_id | company_name | age | dept------------+--------------+-----+------------ 1 | Paul | 32 | IT Billing 2 | Allen | 25 | Engineering 7 | James | 24 | Finance 3 | Teddy | 23 | Engineering 4 | Mark | 25 | Finance 5 | David | 27 | Engineering 6 | Kim | 22 | Finance(7 rows)