7.20

Give an example of a relation schema \(R\) and a set of dependencies such that \(R\) is in BCNF but is not in 4NF.

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There are, of course, an infinite number of such examples. We show the simplest one here.

Let \(R\) be the schema \((A,B,C)\) with the only nontrivial dependency being \(A \twoheadrightarrow B\)