Home Work

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  1. #1
    Its not rocket science vasanth's Avatar
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    Home Work

    Hi Everyone,
    Ok this is my home work.. but i am not gona ask you guys to do it..

    It is for the database class... I have modeled a Entity Relation Diagram (ER Diagram) for a given problem for which a database has to be created... I want to get the model right before i proceed to the next step.. i have created a model which i think is ok.. but i would like you guys to have a look and recommend any modifications which might improve the model.. or a better way of doing it..


    thanx in advance
    bye
    Vasanth
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  2. #2
    and the hat of wrongness Salem's Avatar
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    Well your double ended relationships seem wrong (and confusing)

    For example
    Code:
    +============+                   +============+
    |            | 1               + |            |
    |   Site     |-------------------| Grid Ref   |
    |            |                   |            |
    +============+                   +============+
    A site is composed of many grid references
    A grid reference belongs to one site

    Depending on what your local conventions are on multiplicity, you might consider
    {m,n} - between m and n
    {m,} - at least m
    {,n} - at most n
    * - zero or more, same as {0,}
    + - one or more, same as {1,}


    For me, I would remove Species_Ref_No from Site_record (a site record will contain many species, so it can't really be here), and remove SiteID from Site_Species_Record, and replace it with a Site_record (so you can easily get date and recorder).

    So it would read
    A Site has many Site Records
    A Site Record has many Site Species Records
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  3. #3
    Its not rocket science vasanth's Avatar
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    thanks... for the valuable response.. i am working on that now.. I think a grid reference can belong to more than one site since sites overlap..

    any way will post the new model.. hope to get some review again..

    thanks

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