PostgreSQL UNION 操作符

PostgreSQL UNION 操作符合并两个或多个 SELECT 语句的结果。

UNION 操作符用于合并两个或多个 SELECT 语句的结果集。

请注意，UNION 内部的每个 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每个 SELECT 语句中的列的顺序必须相同。

语法

UNIONS 基础语法如下：

SELECT column1 [, column2 ]

FROM table1 [, table2 ]

[WHERE condition]



UNION



SELECT column1 [, column2 ]

FROM table1 [, table2 ]

[WHERE condition]

这里的条件语句可以根据您的需要设置任何表达式。

实例

创建 COMPANY 表（下载 COMPANY SQL 文件 ），数据内容如下：

db# 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 DEPARTMENT SQL 文件 ），数据内容如下：

db=# 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，我们在 SELECT 语句中使用 UNION 子句将两张表连接起来，如下所示：

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;

得到结果如下：

 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 操作符可以连接两个有重复行的 SELECT 语句，默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。

语法

UINON ALL 子句基础语法如下：

SELECT column1 [, column2 ]

FROM table1 [, table2 ]

[WHERE condition]



UNION ALL



SELECT column1 [, column2 ]

FROM table1 [, table2 ]

[WHERE condition]

这里的条件语句可以根据您的需要设置任何表达式。

实例

现在，让我们把上面提到的两张表用 SELECT 语句结合 UNION ALL 子句连接起来：

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;

得到结果如下：

 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)