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  1. #1
    Join Date
    May 2012
    Posts
    2

    Question Identifying functional dependencies with sample relation

    I'm having trouble understanding the logic in use for identifying functional dependancies.

    Looking at the sample relation below (see link), I understand fd1 - fd3. But when I look at fd4 and fd5, its logic makes me believe that fd6 and fd7 would also be possible, but it's not according to the book I'm studying.

    The logic that operates fd4 and fd5 is to me:

    We conclude that unique combination of values in in columns A and B such as (a, b) is associated with a single value in column E, which in this example is "q". In other words attributes (A, B) functionally determines attribute E, and this is shown as fd4 in the sample relation. We also conclude that attributes (B, C) functionally determine attribute E using the same reasoning described earlier, and this functional dependancy is shown as fd 5 in the sample relation.
    So why is fd6 and fd7 not true?

    Here's the link to the sample relation:

    http://i.imgur.com/xfdWfW7.png

  2. #2
    Join Date
    Feb 2004
    Location
    In front of the computer
    Posts
    15,579
    fd6 and fd7 are different functional dependencies than fd4 and fd5. There are different logical rules, and those rules must not describe a dependency (or I suppose that there could be an error in your book, but that seems unlikely).

    -PatP
    In theory, theory and practice are identical. In practice, theory and practice are unrelated.

  3. #3
    Join Date
    Oct 2014
    Posts
    5
    Functional dependencies FD6 and FD7 are not actually FALSE but wrong (also redundant). This is because FD6's key (i.e. the attributes which imply A) is a true subset of FD2's key. Same applies for FD7 and FD4 and FD5.

    You must remember that the key which implies other attributes must be a) a one by which unique values can be identified but also b) undividable so that no true subset of the key can identify distinct values.

    If this wasn't a clear explanation, I'll try to give a real life example. Say that you have a relation R with attributes (SSN, NAME, ADDRESS). You can identify each person by their SSN (social security number), so FD {SSN} -> {NAME, ADDRESS} holds TRUE.

    But one could argue (like you did with your relation) that you can identify ADDRESS with the combination of {SSN, NAME}, which is of course true in real life, but you don't NEED the NAME to identify a person's address - the SSN is enough.

    Hope this helped.

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