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  1. #1
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    Sep 2008
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    Unanswered: Conceptual ques on keys

    Please help me with this question :

    Consider a relation R (A, B, C, D, E). The only information that you know about R is that ABC and CD are keys. List all possible keys (some will not be compatible with others).

    Thanks

  2. #2
    Join Date
    Dec 2007
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    Quote Originally Posted by lenovo
    Please help me with this question :

    Consider a relation R (A, B, C, D, E). The only information that you know about R is that ABC and CD are keys. List all possible keys (some will not be compatible with others).

    Thanks
    My guess is that "key" in this context means "super key" because that's the only way I can make sense of what's being asked. If what you quoted is verbatim then I suggest you read "key" as "super key". Does that help?

  3. #3
    Join Date
    Sep 2008
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    Thanks for replying ........ This is one answer I got on internet.........but could not understand. But then I think my question will be solved on these lines ...............


    Consider a relation R(A,B,C,D,E,F). The only thing you know about R is that ABCD and
    EF are keys. What is the maximum number of keys that R can possibly have (including ABCD and
    EF)? Explain your calculation.
    Since ABCD and EF are keys, no subset or superset can be a key (recall that a key is
    minimal). The largest number of keys can be found by add an E or an F to selected subsets
    of ABCD. The largest number of subsets of ABCD are of length 2, i.e., 4
    2 = 6 (namely, AB,
    AC, AD, BC, BD, CD). To these we can add either an E or an F to obtain a new key. So the
    total number of keys that are not sub/supersets of ABCD or EF are 12 (namely, ABE, ABF,
    ACE, ACF, ADE, ADF, BCE, BCF, BDE, BDF, CDE, CDF). Adding the two original keys,we have a maximum of 14 possible keys for the relation.



    Any suggestion for mine?

  4. #4
    Join Date
    Dec 2007
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    The answer you gave assumes that "key" means "candidate key", ie a minimal set. That is the usual meaning of "key" and may have been the intention of the question. I interpreted the question as "super key" only because it seemed like a more useful exercise but perhaps I was trying to be too clever.

    Possibly you could impress your teacher by suggesting both interpretations. I've no intention of doing your homework for you though.

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