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We've been getting a number of this type of question lately: short,
and simple on the surface. Short questions are wonderful, because they
leave more room for the answer. Simple questions are even better,
as long as they aren't too simple.
In this case, it's a good question but it might have been too simple.
A candidate key is any key that could be used as the primary key.
This comes with two requirements: it cannot be null, and its values
must be unique.
A composite key contains more than one column.
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