The PostgreSQL UNION provision/administrator is utilized to join the aftereffects of at least two SELECT explanations without restoring any copy columns.
To utilize UNION, each SELECT should have similar number of segments chosen, similar number of segment articulations, a similar information type, and have them in a similar request however they don't need to be a similar length.
Syntax
The fundamental linguistic structure of UNION is as per the following −
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
UNION
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
Here, given condition could be any given articulation dependent on your prerequisite.
Example
Think about the accompanying two tables, (a) COMPANY table is as per the following −
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)
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)
Presently let us join these two tables utilizing SELECT articulation alongside UNION proviso as follows −
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 deliver the accompanying outcome −
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)
The UNION ALL Clause
The UNION ALL administrator is utilized to consolidate the consequences of two SELECT explanations including copy lines. The very guidelines that apply to UNION apply to the UNION ALL administrator too.
Syntax
The fundamental linguistic structure of UNION ALL is as per the following −
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
UNION ALL
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
Here, given condition could be any given articulation dependent on your necessity.
Example
Presently, let us join previously mentioned two tables in our SELECT assertion as follows −
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;
This would create the accompanying outcome −
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)