SQL Set Operator

What
SQL set operators allow us to perform operations between two SQL select sets. The possible ones are:
 * Intersect
 * Union
 * Except

Except
This is the set difference operator as seen in relational algebra. It keeps all elements that are in the first set but not in the second set. It removes elements from the first set that the two sets share.

Union
The union operator combines the two sets. The result will be tuples that are in either sets or both.

Intersect
The intersect operator keeps only tuples that are present in both sets.