postgresql数据库-PostgreSQL UNIONS子句

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PostgreSQL的UNION子句/运算符用于合并两个或多个SELECT语句的结果,不返回任何重复的行。

直接使用UNION,每个SELECT选择的列数必须具有相同的,相同数目的列表达式相同的数据类型,并让它们在相同的顺序,但它们不必是相同的长度。

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]

这里给定的条件可以是根据需要的任何表达式。

例子:

考虑以下两个表: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) 

另一表是DEPARTMENT 如下:

 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,让我们加入这两个表使用SELECT语句连同UNION子句如下:

 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;

This would produce following result:

  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运算符是用来结合两个SELECT语句,包括重复行的结果。联盟适用同样的规则适用于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]

这里给定的条件可以是任何表达式,根据需要。

例子:

现在,让我们携手上述两个表中的SELECT语句如下:

 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)

  

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