Question

If S is a superkey such that S $\cap$ UID is NULL then S $\cup$ UID is also a superkey

Answer 1

From statement "If S is a superkey such that {S ∩ UID} is NULL then {S ∪ UID} is also a superkey". If i delete "{S ∩ UID} is NULL" then also statement is true. Anyway Superkey ∪ anything = Superkey only. What is d use of "{S ∩ UID} is NULL" ??