Sql-server – Implementing foreign keys on generalized data model

database-designforeign keygeneralizationprimary-keysql server

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? 

mammal
 |
 +- wild
 |  |
 |  +- raccoon
 |  |
 |  +- white tail deer
 |
 +- domestic
    |
    +- cat
    |   |
    |   +- siberian
    |
    +- dog
    |   |
    |   +- golden retriever
    |   |
    |   +- lab
    |
    +- raccoon

Best Answer

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:

CREATE TABLE raccon
( 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)

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