Definition
IS-A Relation
The IS-A Relation describes a hierarchical relationship between classes, entities, or concepts. The IS-A relation is used to represent inheritance, categorisation, and generalisation.
Properties
Partial
Total
Disjunctive
An object following a disjunctive IS-A relationship cannot be more than one specialisation at a time.
Conjunctive
An object following a disjunctive IS-A relationship can be more than one specialisation at a time.
Relational Transformation
Generalisation (Roll-up)
Embedding the attributes of two or more specialisations into a single one. If the specialisations are disjunctive, the unused values are set to null.
Specialisation (Roll-down)
Separating the specialisations into two or more tables whereby the the tables must be linked, e.g. through equal primary keys.