The rules for extracting a relational data schema from the class diagram of an application are an extension of the rules for mapping Extended Entity Relationship diagrams onto relational data schemes . A sample of the these rules is the mapping of Generalization/Specialization hierarchies.
A generalization/specialization hierarchy can be of different types depending upon two properties: disjunction/overlapping and partial/total . Their combination yields the following four types of hierarchies: 1) disjoint/total, 2) disjoint/partial, 3) overlapping/total, 4) overlapping/partial. If a generalization/specialization hierarchy of type 2, 3, or 4 has superclass with attributes and subclasses , then it is mapped by using the following rules:
As for generalization/specialization hierarchies of type 1, these are mapped by generating relational tables only for the subclasses with columns attributes of corresponding to the attributes of the subclasses, plus their OIDs, plus the attributes of the superclass.
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