Union is to select all records from two table but common record only once.

Select * from tbemp1
UNION
Select * from tbemp2


Inersect is to select common records from two tables.

Select * from tbemp1
INTERSECT
Select * from tbemp2

Except(Minus) is equivalent to oracle minus clause. Select records of table 1 which are not in table 2.

Select * from tbemp1
EXCEPT
Select * from tbemp2