What is the difference between UNION and UNION ALL in PostgreSQL?

UNION and UNION ALL are both used to combine the result sets of two or more SELECT statements into a single result.

UNION will remove duplicate rows from the result set, whereas UNION ALL will include duplicates

#Example

Table_A

ID	Name
1	Alice
2	Bob
3	Carol

Table_B

ID	Name
2	Bob
3	Carol
4	David

SELECT * FROM Table_A
UNION
SELECT * FROM TABLE_B

gives:

ID	Name
1	Alice
2	Bob
3	Carol
4	David

SELECT * FROM Table_A
UNION ALL
SELECT * FROM TABLE_B

gives (notice duplicates):

ID	Name
1	Alice
2	Bob
3	Carol
2	Bob
3	Carol
4	David