Alias 语法

你可以通过给出另一个名字来临时重命名一个表或一个列，这就是所谓的ALIAS。使用表的别名意味着在一个特定的PostgreSQL语句中重命名一个表。重命名是一个临时的改变，实际的表名在数据库中不会改变。

列别名是用来为一个特定的PostgreSQL查询重命名一个表的列。

语法

table别名的基本语法如下 −

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

column别名的基本语法如下 −

SELECT column_name AS alias_name
FROM table_name
WHERE [condition];

示例

考虑以下两个表，(a) COMPANY表如下所示 −

testdb=# 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)

(b) 另一个表是DEPARTMENT，如下所示 −

 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)

现在，以下是TABLE ALIAS的用法，我们用 C 和 D 分别作为 COMPANY 和 DEPARTMENT 表的化名。 −

testdb=# SELECT C.ID, C.NAME, C.AGE, D.DEPT
   FROM COMPANY AS C, DEPARTMENT AS D
   WHERE  C.ID = D.EMP_ID;

上面给出的 PostgreSQL 语句将产生以下结果 −

 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)

让我们看一个使用COLUMN ALIAS的例子，其中COMPANY_ID是ID列的别名，COMPANY_NAME是name列的别名。 −

testdb=# 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;

上面给出的 PostgreSQL 语句将产生以下结果 −

 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)