PostgreSQL UNION ALL

UNION ALL operator is used to combine the results of two SELECT statements including 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 ALL clause shows all rows including duplicate rows

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