Inner Join, Outer Join and Union

Inner Join: Inner join is a way of combining two relations by matching a tuple of a relation with corresponding tuple in next relation, only when they have a common value in a particular attribute.

Outer Join: Outer join is a way of combining two relations by matching a tuple of a relation with corresponding tuple in next relation, but it also includes tuples which have no match in other relation. The tuple which exist on only one relation take null value in remaining attributes. An outer join is either a left-outer join or a right-outer join based on whether all items are taken from relation on left or relation on right respectively.

Union: Union is a way of combining two relations by taking all the elements of both relations.  

Take the following example:

Relation A:

Name	Age
Randy	16
Nathan	12
Sunny	15

Relation B:

Name	Grade
Randy	11
Nathan	7
Yogi	10
Rachel	8

Union of A and B:

Name	Age	Name	Grade
Randy	16	Randy	11
Nathan	12	Nathan	7
Sunny	15	Sunny	null
Yogi	null	Yogi	10
Rachel	null	Rachel	8

*union has all elements from both relations.

Inner Join:

Name	Age	Name	Grade
Randy	16	Randy	11
Nathan	12	Nathan	7

*inner join has only elements which can be matched from A to B.

Outer Join:

Name	Age	Name	Grade
Randy	16	Randy	11
Nathan	12	Nathan	7
Yogi	null	Yogi	10
Rachel	null	Rachel	8

*it has even elements which are not in A and this outer join is specifically a right outer join as it includes all items from relation B which we assume to be on the right.