Cos the key of UNION in T-SQL gets rid of the replicated records,so : The sizes of Q1 always >= The size of Q2 Case 1) : if there is NOT replicated records in R(a,b) then Size(Q1) = Size(Q2) Case 2): if there contains replicated records in R(a,b) then Size(Q1) > Size(Q2) to sum up, sizes(Q1) >= sizes(Q2) So, the final answer is C
The sizes of Q1 always >= The size of Q2
Case 1) : if there is NOT replicated records in R(a,b) then Size(Q1) = Size(Q2)
Case 2): if there contains replicated records in R(a,b) then Size(Q1) > Size(Q2)
to sum up, sizes(Q1) >= sizes(Q2)
So, the final answer is C