Entities, attributes, relationships, identifiers, and foreign keys are important data modeling constructs, particularly for relational databases, but subtypes and supertypes (S/Stypes) are even more important. Relational DBMSs do not handle S/Stypes. However, many data modeling CASE tools do include provision for S/Stypes, so data modelers need to understand how to properly use these constructs.

S/Stypes are truly essential for the data modeler (Graeme Simsion covered it early in his book, Data Modeling Essentials). They allow the database designer to defer choosing what objects to materialize or what (relational) tables to build until later in the database development process.

In the meantime, S/Stypes offer a means to formally represent overlapping entity/object populations at the very beginning of the design process. If you are not using S/Stypes in your data modeling, or want to deepen your understanding of these constructs, this session is for you. Knowledge and use of subtypes/supertypes should be in every data modeler's toolkit.

• Situations which motivate the need for another data modeling construct
• Definition of subtypes and supertypes - characteristics of a subtype-supertype "relationship"
• Abstraction mechanisms: specialization vs. generalization; attribute vs. entity abstraction
• Required conditions when using S/Stypes
• The "universal relation" – the ultimate supertype (vs. no entity types at all!)
• Distinguishing attribute(s) on the supertype: intensional vs. extensional sets
• Declaring constraints on a subtype-supertype relationship
• Inheritance in data models vs. inheritance and reuse in object-oriented design (they are not the same); inheritance priority, blocking, and overriding.