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I have a problem understanding a particular case of BCNF. Let's suppose that we have a relation R(B, C, D) with df:{C->D}. Is my relation in BCNF and if yes why??? Is C considered as superkey?

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The only key is {BC}. You have a partial key dependency: C->D. Since you have a partial key dependency, R is not in 2NF. So R is also not in BCNF.

An informal way of expressing BCNF is that every arrow in every FD is an arrow that comes out of a candidate key. In R, the arrow in "C->D" doesn't come out of a candidate key.

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