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PostgreSQL UNION 操作符

PostgreSQL UNION 操作符

日期:2022-07-07

本文字数:310 字 阅读完需:约 2 分钟

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:

1SELECT column1 [, column2 ] 2FROM table1 [, table2 ] 3[WHERE condition] 4 5UNION 6 7SELECT column1 [, column2 ] 8FROM table1 [, table2 ] 9[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:

1runoobdb# select * from COMPANY; 2 id | name | age | address | salary 3----+-------+-----+-----------+-------- 4 1 | Paul | 32 | California| 20000 5 2 | Allen | 25 | Texas | 15000 6 3 | Teddy | 23 | Norway | 20000 7 4 | Mark | 25 | Rich-Mond | 65000 8 5 | David | 27 | Texas | 85000 9 6 | Kim | 22 | South-Hall| 45000 10 7 | James | 24 | Houston | 10000 11(7 rows)

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

1runoobdb=# SELECT * from DEPARTMENT; 2 id | dept | emp_id 3----+-------------+-------- 4 1 | IT Billing | 1 5 2 | Engineering | 2 6 3 | Finance | 7 7 4 | Engineering | 3 8 5 | Finance | 4 9 6 | Engineering | 5 10 7 | Finance | 6 11(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:

1SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT 2 ON COMPANY.ID = DEPARTMENT.EMP_ID 3 UNION 4SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT 5 ON COMPANY.ID = DEPARTMENT.EMP_ID;

得到结果如下:

1emp_id | name | dept 2--------+-------+-------------- 3 5 | David | Engineering 4 6 | Kim | Finance 5 2 | Allen | Engineering 6 3 | Teddy | Engineering 7 4 | Mark | Finance 8 1 | Paul | IT Billing 9 7 | James | Finance 10(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:

1SELECT column1 [, column2 ] 2FROM table1 [, table2 ] 3[WHERE condition] 4 5UNION ALL 6 7SELECT column1 [, column2 ] 8FROM table1 [, table2 ] 9[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:

1SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT 2 ON COMPANY.ID = DEPARTMENT.EMP_ID 3 UNION ALL 4SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT 5 ON COMPANY.ID = DEPARTMENT.EMP_ID;

得到结果如下:

1emp_id | name | dept 2--------+-------+-------------- 3 1 | Paul | IT Billing 4 2 | Allen | Engineering 5 7 | James | Finance 6 3 | Teddy | Engineering 7 4 | Mark | Finance 8 5 | David | Engineering 9 6 | Kim | Finance 10 1 | Paul | IT Billing 11 2 | Allen | Engineering 12 7 | James | Finance 13 3 | Teddy | Engineering 14 4 | Mark | Finance 15 5 | David | Engineering 16 6 | Kim | Finance 17(14 rows)