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I'm doing a relational design course on Stanford open courseware. I understand why functional dependencies are named that way - A->B means B is specified when A is specified, or in other words, the value of B functionally depends on the value of A. But I'm not able to think of a simple reason why multivalued dependencies are named that way. Understanding this would make it much easier to remember and use the concept.

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A multivalued dependency A->->B means that each value of A determines a set of values of B (and not a single value of B as in functional dependencies).

For instance, suppose that programmers have an attribute programmer-id and an attribute known-language, and each programmer can know several languages, you have the multivalued dependency:

programmer-id ->-> know-language

So, supposing that the programmer with program-id 7 knows SQL and Ruby, this means that in a table in which you have both attributes programmer-id and know-language, every time there is program-id 7 there must be two different rows, one with language 'SQL' and one with language Ruby, and all the other attributes equal:

programmer-id  know-language other-attribute1 other-attribute2 ...
    7              SQL           XXX               YYY
    7             Ruby           XXX               YYY
    8             Java           AAA               BBB
 ...
  • Re "each value of A determines a set of values of B" & "not a single value of B as in FDs" That might be a mnemonic if you already know what a MVD is, but it isn't a clear description let alone definition. Eg the MVD "determines" means a different (& here undefined) thing than the FD "determines". – philipxy Jul 22 '16 at 12:15

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