Meaning of the is unique clause here Suppose I have two relations R and S and I execute the following query:

Select R.* from R,S where R.a=S.a and is unique R

What is the effect of the is unique clause here ?
I would say "is unique" is an operator that returns true if, and only if. the tested SQL relation does not contain any duplicate tuples.

For context, this is question 11 in this quiz, and Q54 in GATE 2014 Paper 2 (pdf).

Answer D is therefore incorrect, because it returns an empty (null) result if R contains any duplicate tuples.