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Asked: 2026-06-11T04:37:41+00:00

I have a quick question regarding candidate keys and superkeys. Say you have two

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I have a quick question regarding candidate keys and superkeys. Say you have two keys (a, b) where ‘a’ is a primary key and b is a candidate key. Would the combination of these two keys be a superkey ie. would (a,b) be a superkey? Or would it be a candidate key. My assumption is that it would be a superkey because the definition of a candidate key states that it is a irreducible superkey and the combination of the two fields a and b could be reduced to either a or b. Is this logic correct? Or am I missing something here? Thanks!

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  1. Editorial Team

    Would the combination of these two keys be a superkey ie. would (a,b) be a superkey?

    Yes, it would still uniquely identify rows.

    Or would it be a candidate key.

    No, it would no longer be minimal.

    My assumption is that it would be a superkey because the definition of a candidate key states that it is a irreducible superkey and the combination of the two fields a and b could be reduced to either a or b. Is this logic correct?

    Almost. Yes it would be a superkey, but not because it can be reduced. It would be a superkey because it is unique.

    Every candidate key is superkey, but not every superkey is candidate key. So {a} is both candidate and superkey, {b} is both candidate and superkey and {a, b} is just superkey.

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