ER/Studio - Embarcadero Technologies Product Documentation

USING ER/STUDIO > DEVELOPING THE LOGICAL MODEL
See Also
Creating and Editing Data Base Views
Creating Subtype Cluster Relationships
NOTE: Subtype cluster relationships can only be represented in the logical model. When the physical
model is generated from a logical model, the subtype relationship is converted to identifying
relationship.
A subtype cluster refers to a group of entities sharing similar characteristics that form a generalization hierarchy. In
each subtype cluster, there is one parent entity, known as a supertype, and one or more subtype entities. The
supertype generalizes a set of subtypes; the common characteristics of subtype entities are assigned to the supertype
while the subtype entities have more specific attributes. The subtypes, also known as category entities, represent
homogeneous subsets of the supertype. The attributes and relationships of the supertype entity are propagated to all
of its subtypes. Discriminators, an attribute of the subtype, distinguish the entities in the subtype from each other.
Other than recognizing a generalization hierarchy, an important issue with subtyping is to decide how to implement a
subtype cluster in the physical model. Sometimes it is best to project subtypes into the supertype, representing the
entire cluster as a single, generalized entity. Other times, you may need the granularity of separate subtype entities,
choosing to implement the subtypes and supertype one-for-one or rolling the supertype down into the subtypes.
During the logical model phase, you should search for hierarchies that can reveal a more general approach to
aggregating attributes into entities. By addressing the general case, you can design a more flexible and extensible
approach to storing data. The use of subtyping is a powerful tool in identifying and portraying such generalizations.
EMBARCADERO TECHNOLOGIES > ER/STUDIO® 8.0.3 USER GUIDE 137