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  1. #1
    Join Date
    Jun 2007
    Posts
    3

    Question data modelling question

    hi,

    I am learning datamodeling basics. I am seriously confused in the concept of entity types.

    Independent/dependent entities and parent/child entities. are they synonymous?

    if not then whats the difference?

    thanks
    Hamza.

  2. #2
    Join Date
    Apr 2002
    Location
    Toronto, Canada
    Posts
    20,002
    independent entities have their own primary keys

    dependent entities have a primary key in which part of it is a foreign key to some other table

    a dependent entity is therefore a child entity of at least one other entity

    independent entities can be child entities, too, in which case the foreign key is not part of the primary key

    rudy.ca | @rudydotca
    Buy my SitePoint book: Simply SQL

  3. #3
    Join Date
    Jun 2007
    Posts
    3
    Hi 937,

    Thanks for your help. I think many things are clear now, below is summary of my understanding. Please help if its wrong somewhere.

    Parent entity- From which the primary key migrates.
    Child entity- To which the primary key migrates and becomes a foreign key.


    Dependent entities- Which do not have their own primary keys and rely on the keys of the independent entities they are related to.
    Dependant entities also called 'Weak entities' and 'ID dependant entities'


    • If an independent entity is a child entity, it has its own primary key and the imported foreign key is not the primary key.


    In 1:M relationship, is it true that the entity occuring once is always the Independant entity?

    In a case where one is dependant and one is independant, is it ever possible that the dependant can be the Parent?

    Thanks.
    Hamza.
    Last edited by hamzaali; 07-01-07 at 07:01.

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