In the approach to entity/relationship modeling described in this book, there are three constraints on the treatment of super-/sub-types:
· Completeness - Each instance of the super-type must be an instance of one of the sub-types. This is equivalent in UML to calling the super-type “abstract”. That is, in UML you can impose this constraint or not. In the version of entity/relationship modeling described here, the constraint always applies, but it can be finessed by adding a sub-type Other… (Other Organization, Other Geographic Area, etc.).
· Exclusivity – No instance of the super-type may be an instance of more than one of the sub-types.
· No multiple inheritance – Each sub-type may have only one super-type. ...