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I have been studying DBMS ans I have a question that I got conflicting answers to: Is prime attribute that is dependant on another prime attribute allowed under BCNF?

Meaning P->P.

I know that NP->P is not allowed. and also NP->NP (as it is not 3NF - transitive relation) Thanks.

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By definition, a relation is in BCNF if and only if, for every non trivial functional dependency, the left hand part is a candidate key.

So, if you have a dependency A → B where both A and B are prime attributes, this dependency violates the Normal Form only if A is not a candidate key, but a part of some candidate key.

Note that in that case, the dependency does not violates the 3NF.

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