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sqldatabasefunctional-dependencies

Implied functional dependencies?

I encountered this question but I can't get my head around to understand what

implied functional dependencies

mean? Original question is:

List all the functional dependencies found in the spreadsheet. Remove any trivial or implied functional dependencies. Use FD rules if necessary to minimize the number of FDs needed to cover all FDs.

Now I know how to find functional dependencies and what trivial dependencies are but what are implied functional dependencies? I tried to google but got no results. Could anyone explain? Thank you.

Solution

  • Start with these explicit functional dependencies.

        A->B
    B->C

    A->B and B->C, therefore A->C (transitivity rule, from Armstrong's axioms).

    The FD A->C is an implicit FD. It's logically implied by the two explicit FDs A->B and B->C.