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  1. #1
    Join Date
    Nov 2011

    a simple looking ERD (chen notation) situation - but seems quite tricky

    Hi guys! I'm new to this forum, I originally joined, because of this problem I am having. I am a student studying databases applications and technologies, and it's one of my favourite courses at the moment. I think you will be seeing a fair bit of me round here, and when I'm less busy I'll keep a look out to give help with SQL and DB design concepts where possible

    Anyway, the problem:

    whilst designing the conceptual model for a database system, I've bumped into a point where I can go no further. :/

    Here's the portion of the ERD I am stuck with;

    The above is wrong because:

    I need each occurrence of B to mandatorily have a relationship with at least one occurrence in either A1, or A2, or both, but it can't just be entered with having no relationships in both of A1 an A2.

    How do I express this in chen notation? because in the above diagram, it just shows that I could make an occurrence in B that has no relationship with either of A1, or A2, because there's no double "mandatory" lines leading from it

    Oh, also, I need the solution to still use a superclass/subclass relationship at least somewhere (it's brownie points)

    help would be brilliant
    Last edited by pb910; 12-05-11 at 09:39.

  2. #2
    Join Date
    Nov 2011
    ok I didn't this before, here's the solution:

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