All Candidate keys are Superkeys whereas all Superkeys are not Candidate keys. Why? [duplicate]

This question already has answers here: Closed 12 years ago. 开发者_StackOverflow

Possible Duplicates:

What's the point of a candidate key?

Superkey vs. Candidate key

Thanks in advance.

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I think the idea of homework is that you do it yourself. Anyway, Wikipedia supplies a clue