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  1. #1
    Join Date
    Oct 2007

    When to use Associative Entity

    Hello All,

    I have been struggling with this question, searched and looked at books and read lots, but still not sure what the best answer would be.

    Basically I am creating a database for a project. It is an estate agent's database and while writing the data dictionary and putting together an ER diagram I am unsure about a particular entity.

    Property for sale is obviously a strong entity
    Prospective Buyer is too
    Branch too
    Employee too.

    However buyers make enquiries, these are registered by an employee, about one or more properties. The information needed regarding enquiries comprises the time of the enquiry, details of the employee who dealt with it, notes on the enquiry and the comments on the outcome. This means there is a three way relationship.

    I was thinking it would be correct to add Enquiry as an associative entity, as this would resolve the many to many relationship between property and prospective buyer. However I was also thinking that I could use enquiry as a strong entity, even though it is not something that exists until a buyer makes it. Maybe I have just answered my own question, but would welcome confirmation. In that case maybe I should include it in the relationship part of the data dictionary as the relationship between buyer and property to begin with

    I know that the forum is not for homework, but I have tangled with this thoroughly and would welcome a bit of perspective from others!
    Last edited by beechfielder; 12-13-07 at 08:38.

  2. #2
    Join Date
    May 2005
    San Antonio, Texas
    hehe.. I had to look strong entity up.

    For your purposes, you can probably answer your own question with this:

    Can an enquiry exist on its own? (you did already answer that)
    Vi veri veniversum vivus vici
    By the power of truth, I, a living man, have conquered the universe

  3. #3
    Join Date
    Dec 2007
    I'm working on an identical problem on a course that i'm doing :-)

    Yes I think it is an assosiative entity, it relies upon buyer, propety and an employee handling the enquirey.

    What i'm confused about is the participation between these entities i.e. mandatory, optional, none, one or many?

    Any ideas on that?

  4. #4
    Join Date
    Oct 2007
    The relationsip is mandatory for both buyer and enquiry I think, as a buyer is does not exist until they have made an enquiry. For the employee, enquiry relationship the relationship is optional for the enquiry, as an employee does not have to have an enquiry, but the enquiry must be registered. For property, enquiry it is optional for enquiry side. A property can exist without an enquiry, but for an enquiry to exist there must be a property, so the property side is mandatory.

    That is if you are doing the same course of course :-) Yours may be different!

  5. #5
    Join Date
    Oct 2007
    Also one to many for all three, with the many being on the enquiry side

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