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📜  PostgreSQL-ALIAS语法

📅  最后修改于: 2020-11-26 06:29:29             🧑  作者: Mango

您可以通过提供另一个名称ALIAS暂时重命名表或列。表别名的使用意味着在特定的PostgreSQL语句中重命名表。重命名是一个临时更改，数据库中的实际表名不会更改。

列别名用于重命名表的列，以用于特定的PostgreSQL查询。

句法

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];

例

考虑以下两个表，(a) COMPANY表如下-

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)另一个表格是部门—

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，下面是表别名的用法，我们分别使用C和D作为COMPANY表和DEPARTMENT表的别名-

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的示例，其中COMPANY_ID是ID列的别名，而COMPANY_NAME是名称列的别名-

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)