Given two relations R1(A, B) and R2(C, D), the result of following query
Select distinct A, B
from R1, R2
is guaranteed to be same as R1 provided one of the following condition is satisfied.
1
R1 has no duplicates and R2 is empty.
2
R1 has no duplicates and R2 is non-empty.
3
Both R1 and R2 have no duplicates.
4
R2 has no duplicates and R1 is non - empty