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Suppose the ASCII tree below is a UML generalized concept model. To represent these entities/classes in UML, lets assume [mammal] a super class, and the level 1 classes, [wild] and [domestic] are sub classes in UML. In other words, class [wild] inherits all attributes from [mammal], and [domestic] inherits all attributes from [mammal] as well. Furthermore, [raccoon] and [white tail deer] inherit all attributes from [wild].

I want to convert the UML model to a relational database model.

Notice how [raccoon] extends both [wild] and [domestic]. In other words, it has two parents. I would normally make the primary key of all descendant tables the same as their parent primary key. In addition, I'd make the foreign key in the descendant table the same as the primary key in its parent table. Because [raccoon] has two parents, how would I build that table?

 +- wild
 |  |
 |  +- raccoon
 |  |
 |  +- white tail deer
 +- domestic
    +- cat
    |   |
    |   +- siberian
    +- dog
    |   |
    |   +- golden retriever
    |   |
    |   +- lab
    +- raccoon 
share|improve this question
I think having 2 FKs from racoon(pk), one referencing wild(pk) and the other referencing domestic(pk) would solve this. Right? – ypercubeᵀᴹ Nov 20 '13 at 9:25
That's what I thought too. Would there also be a PK for the sub class (child table) in addition to these two FKs? I can't make it a composite PK because both would have to exist (NOT NULL). – MacGyver Nov 20 '13 at 15:53
I don't see why you would need a composite key here or why that would be impossible. – ypercubeᵀᴹ Nov 20 '13 at 16:00
up vote 3 down vote accepted

You can have two (or more) foreign keys in a table. They can be both regarding the same attribute and referencing different tables. So you could have 2 foreign keys from racoon(pk), one referencing wild(pk) and the other referencing domestic(pk). Something like:

( mammal_id INT  PRIMARY KEY
, FOREIGN KEY (mammal_id)
    REFERENCES domestic (mammal_id)
, FOREIGN KEY (mammal_id)
    REFERENCES wild (mammal_id)
) ;

(assuming all tables have mammal_id as the primary key)

share|improve this answer
Is that Oracle? Can you add SQL Server to your answer so I'm doing it correctly? I forgot to add the tag. – MacGyver Nov 20 '13 at 16:34
The syntax is valid for both Oracle and SQL-Server: SQL-Fiddle – ypercubeᵀᴹ Nov 20 '13 at 16:37
Learn something new every day – MacGyver Nov 20 '13 at 16:43

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