ERD. Modeling Question

 

Hopefully someone may be able to quickly advise the best way for me to model my ERD (I'm using IDEF1X & ERWin not Chen Notation) although it doesn't really matter.

Basically I have Three Entities

  • Organisation
  • Person
  • Trustee

    • Business rules state that a Trustee can be either a person or organisation.

    My question is, would it be correct to have two primary, foreign key relationships to Trustee. One from Organisation > Trustee and the second from Person > Trustee. Where one of the FK fields of Trustee will always be NULL depending on if the Trustee is a person or organisation.

    Or would it be better to have some sort of associative table between person and organisation (person_organisation) and create a PK, FK relationship from person_Organisation > Trustee.

    The second option seems better as there might be other entities that also need to link to both Organisation and person.

    Hopefully that makes sense.

    Many many thanks in advance

    EDIT

    Well, this should work too:

    trustee_model_2

    It seems to me that Trustee is just one example of a role a person can play in an organisation. So really, we're talking about a three-way association Person_Organization_Role where all three are the primary key. Now, that could be broken down into Person_Organization > PO_Role, but my preference is not to take things that far.

    链接地址: http://www.djcxy.com/p/70612.html

    上一篇: 关系数据库设计模式？

    下一篇: ERD。 建模问题