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Thread: Lossless Join

  1. #1
    Join Date
    May 2011
    Posts
    2

    Lossless Join

    Hello,
    I want to know if the following is an example of lossless decomposition, and why/why not?

    R A B C
    1 A 1000
    2 B 2000
    1 C 1000

    The above is decomposed into:


    A C
    1 1000
    2 2000
    1 1000 and


    B C
    A 1000
    B 2000
    C 1000.

    Will this be lossless? If so, why? If not, why not?



    Thanks,
    Prasanna

  2. #2
    Join Date
    Mar 2003
    Location
    The Bottom of The Barrel
    Posts
    6,102
    First I would start out by figuring out what lossless decomposition is.
    oh yeah... documentation... I have heard of that.

    *** What Do You Want In The MS Access Forum? ***

  3. #3
    Join Date
    May 2011
    Posts
    2

    Lossless Join

    Thanks for the reply. I understand what a lossless join is, but I am just confused with these types of examples. This particular example, on joining the 2 decomposed tables, it gives duplicate rows, but not rows that didn't exist at all in the original table.

    It just gives rows which existed more than 1 time. Would it still be lossless?

    Thanks,
    Prasanna

  4. #4
    Join Date
    May 2011
    Posts
    1

    Lossless Join

    It is a lossless join. Small world!!

  5. #5
    Join Date
    Dec 2007
    Location
    London, UK
    Posts
    741
    The concept of lossless / lossy joins is part of relational database theory. Join in that context is an operation on relations and not "tables" of any other (non-relational) kind.

    This:

    A C
    1 1000
    2 2000
    1 1000
    looks like something that has duplicate tuples and therefore it isn't a relation and isn't a relational decomposition. If it isn't a relation then what you are describing can't be a join, let alone a lossless one. Of course it's quite possible to define joins and similar operations on tables (such as SQL tables) but then you'd also have to define what a lossless join meant in that same context. I'm not familiar with any such definition.

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