PostgreSQL UNIONS子句

The PostgreSQL UNION 子句/运算符用于将两个或多个SELECT语句的结果合并在一起，而不返回任何重复的行。

要使用UNION，每个SELECT语句必须具有相同数量的选择列、相同数量的列表达式、相同的数据类型，并且它们必须按照相同的顺序排列，但它们的长度可以不同。

语法

UNION的基本语法如下所示 –

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

在这里，给定的条件可以是基于您的要求的任何给定表达式。

示例

考虑以下两个表格，(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 –

testdb=# 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)

现在让我们使用SELECT语句和UNION子句来连接这两个表，如下所示：

testdb=# SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT
   ON COMPANY.ID = DEPARTMENT.EMP_ID
   UNION
      SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT
         ON COMPANY.ID = DEPARTMENT.EMP_ID;

这将产生以下结果 –

emp_id | name  |  dept
--------+-------+--------------
      5 | David | Engineering
      6 | Kim   | Finance
      2 | Allen | Engineering
      3 | Teddy | Engineering
      4 | Mark  | Finance
      1 | Paul  | IT Billing
      7 | James | Finance
(7 rows)

UNION ALL子句

UNION ALL运算符用于合并两个包括重复行的SELECT语句的结果。UNION ALL运算符的应用规则与UNION相同。

语法

UNION ALL 的基本语法如下 −

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION ALL

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

在这里，给定条件可以是根据您的要求的任何给定表达式。

示例

现在，让我们将上述两个表连接在我们的SELECT语句中，如下所示-

testdb=# SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT
   ON COMPANY.ID = DEPARTMENT.EMP_ID
   UNION ALL
      SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT
         ON COMPANY.ID = DEPARTMENT.EMP_ID;

这将产生以下结果 −

emp_id | name  | dept
--------+-------+--------------
      1 | Paul  | IT Billing
      2 | Allen | Engineering
      7 | James | Finance
      3 | Teddy | Engineering
      4 | Mark  | Finance
      5 | David | Engineering
      6 | Kim   | Finance
      1 | Paul  | IT Billing
      2 | Allen | Engineering
      7 | James | Finance
      3 | Teddy | Engineering
      4 | Mark  | Finance
      5 | David | Engineering
      6 | Kim   | Finance
(14 rows)