UNION
If R1 and R2 are two union compatible relations then R3 = R1 ∪ R2 is the relation having tuples that are either in R1 or in R2 or in both.
In case, R3 will have tuples such that R3 = {t | R1 ∋ t ∨ R2 ∋ t}.
Example
R1
|
X
|
Y
|
|
A1
|
B1
|
|
A7
|
B7
|
|
A2
|
B2
|
|
A4
|
B4
|
|
R2
|
A
|
B
|
|
A1
|
B1
|
|
A2
|
B2
|
|
A3
|
B3
|
|
A4
|
B4
|
R3 = R1 ∪ R2 is
Q
|
A
|
B
|
|
A1
|
B1
|
|
A2
|
B2
|
|
A3
|
B3
|
|
A4
|
B4
|
|
A7
|
B7
|
|
|
Note: 1) Union is a commutative operation, i.e, R ∪ S = S ∪ R.
2) Union is an associative operation, i.e.
R ∪ (S ∪ T) = (R ∪ S) ∪ T.