PostgreSQL UNION

PostgreSQL UNION clause/operator is used to combine the results of two or more SELECT statements without returning any duplicate rows

postgres=# select * from tab1;     postgres=# select * from tab2;
 id | name                              id |  name  
----+-------                           ----+--------
  1 | nijam                              1 | jamal
  2 | abu                                2 | smith
  3 | umar                               4 | daniel
  7 | jamal                              5 | zaheer
(4 rows)                               (4 rows)

-- From above table  tab1 and tab2 having one duplicate values "name=jamal" is duplicate so union clause shows only one rows of tab1 table (7,jamal), table tab2 duplicate row will not show 

postgres=# select name from tab1 union select name from tab2;    
  name  
--------
 smith
 daniel
 zaheer
 nijam
 abu
 jamal
 umar
(7 rows)