3.1 Extracting a Relational Data Schema from the Class Diagram

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  [6]. 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  [6]. 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 $C$ with attributes $a$₁, a₂, ..., a_n and $m$ subclasses $S$₁, S₂, ..., S_m , then it is mapped by using the following rules:

Create a table $T$_C with columns $a$₁, ..., a_n, OID_C corresponding to the attributes of class $C$ plus its OID. The latter will be a candidate primary key for the table $T$_C.

For each subclass $S$_i, , create a table $T$_i with columns $$attributes of $S$_i, OID_Si . Once again, here the OID will be a candidate primary key for the table $T$_i.

Create a system table INHERITANCE, if it does not exist. This table will be updated with the insertion of a new record (superclass-name, subclass-name) for each subclass $S$_i, and will be used for reverse engineering purposes.

As for generalization/specialization hierarchies of type 1, these are mapped by generating relational tables only for the subclasses $S$₁, S₂, ..., S_m with columns $$attributes of $S$_i, OID_Si, a₁, ..., a_n corresponding to the attributes of the subclasses, plus their OIDs, plus the attributes of the superclass.

Next: Refining the Application through the Dynamic Constructs of the Object Model
Up: Mapping Class Diagrams onto Relational Applications
Previous: Mapping Class Diagrams onto Relational Applications