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==== Keys ====
A ''candidate key'', or simply a ''key'', is the smallest [[subset]] of attributes guaranteed to uniquely differentiate each tuple in a relation. Since each tuple in a relation must be unique, every relation necessarily has a key, which may be its complete set of attributes. A relation may have multiple keys, as there may be multiple ways to uniquely differentiate each tuple.<ref name="professionals"/>{{rp|31–33}}
An attribute may be unique across tuples without being a key. For example, a relation describing a company's employees may have two attributes: ID and Name. Even if no employees currently share a name, if it is possible to eventually hire a new employee with the same name as a current employee, the attribute subset {Name} is not a key. Conversely, if the subset {ID} is a key, this means not only that no employees ''currently'' share an ID, but that no employees ''will ever'' share an ID.<ref name="professionals"/>{{rp|31–33}}
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