Question :
If R{abc} is a relation, f{ a->b b->a a->c b->c }, then a and b are candidate keys. What about ab ? Is it a candidate key or is it a super key or both ?
Answer :
The union between A and B is a superkey.
(A candidate key is a superkey but a superkey could be not a candidate key)
Moreover, remember that a candidate key is a particular type of superkey, so you shouldn’t worry about if it is “both” key and superkey.