Prove that every relation has a key.
What will be an ideal response?
Since relations are sets and, thus, cannot have identical elements, the set of all attributes in a relation must be a superkey. If this is not a minimal superkey, some strict subset of it must also be a superkey. Since the number of the attributes in every relation is ?nite, we will eventually get a minimal superkey, i.e., a key of the relation.
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