PostgreSQL UNIONS子句

PostgreSQL UNION子句/运算符用于组合两个或多个SELECT语句的结果，而不返回任何重复的行。

要使用UNION，每个SELECT必须具有相同的列数，相同数量的列表达式，相同的数据类型，并且具有相同的顺序，但不一定要相同。

语法：

UNION的基本语法如下：

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

UNION

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

这里根据您的要求给出的条件表达式。

示例：
考虑以下两个表，COMPANY表如下：

zyiz_db=# SELECT * from COMPANY;
 id | name  | age | address   | salary
----+-------+-----+-----------+--------
| Paul  |  32 | California|  20000
| Allen |  25 | Texas     |  15000
| Teddy |  23 | Norway    |  20000
| Mark  |  25 | Rich-Mond |  65000
| David |  27 | Texas     |  85000
| Kim   |  22 | South-Hall|  45000
| James |  24 | Houston   |  10000
(7 rows)

另一张表是DEPARTMENT如下：

zyiz_db=# SELECT * from DEPARTMENT;
 id | dept        | emp_id
----+-------------+--------
| IT Billing  |      1
| Engineering |      2
| Finance     |      7
| Engineering |      3
| Finance     |      4
| Engineering |      5
| Finance     |      6
(7 rows)

现在使用SELECT语句和UNION子句连接这两个表，如下所示：

zyiz_db=# 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
--------+-------+--------------
| David | Engineering
| Kim   | Finance
| Allen | Engineering
| Teddy | Engineering
| Mark  | Finance
| Paul  | IT Billing
| James | Finance
(7 rows)

UNION ALL子句

UNION ALL运算符用于组合两个SELECT语句(包括重复行)的结果。 适用于UNION的相同规则也适用于UNION ALL运算符。

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]

这里根据您的要求给出的条件表达式。

示例：
现在，我们在SELECT语句中加入上面提到的两个表，如下所示：

zyiz_db=# 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
--------+-------+--------------
| Paul  | IT Billing
| Allen | Engineering
| James | Finance
| Teddy | Engineering
| Mark  | Finance
| David | Engineering
| Kim   | Finance
| Paul  | IT Billing
| Allen | Engineering
| James | Finance
| Teddy | Engineering
| Mark  | Finance
| David | Engineering
| Kim   | Finance
(14 rows)