Information Management (Chapter 4)

Extended entity relationship model (EERM)

Sometimes referred to as the enhanced entity relationship model; the result of adding more semantic constructs, such as entity supertypes, entity subtypes, and entity clustering, to the original entity relationship (ER) model.

EER diagram (EERD)

The entity relationship diagram resulting from the application of extended entity relationship concepts that provide additional semantic content in the ER model.

Entity supertype

In a generalization or specialization hierarchy, a generic entity type that contains the common characteristics of entity subtypes.

Entity subtype

In a generalization or specialization hierarchy, a subset of an entity supertype. The entity supertype contains the common characteristics and the subtypes contain the unique characteristics of each entity.

Two criteria help the designer determine when to use subtypes and supertypes

There must be different, identifiable kinds or types of the entity in the user’s environment.

The different kinds or types of instances should each have one or more attributes that are unique to that kind or type of instance.

Specialization hierarchy

A hierarchy based on the top-down process of identifying lower-level, more specific entity subtypes from a higher-level entity supertype. Specialization is based on grouping unique characteristics and relationships of the subtypes

Inheritance

In the EERD, the property that enables an entity subtype to inherit the attributes and relationships of the entity supertype

Subtype discriminator

The attribute in the supertype entity that determines to which entity subtype each supertype occurrence is related.

Disjoint subtypes

Also known as nonoverlapping subtypes, In a specialization hierarchy, these are unique and nonoverlapping subtype entity set.

Overlapping subtype

In a specialization hierarchy, a condition in which each entity instance (row) of the supertype can appear in more than one subtype.

Completeness constraint

A constraint that specifies whether each entity supertype occurrence must also be a member of at least one subtype. The completeness constraint can be partial or total.

Partial completeness

In a generalization or specialization hierarchy, a condition in which some supertype occurrences might not be members of any subtype.

Total completeness

In a generalization or specialization hierarchy, a condition in which every supertype occurrence must be a member of at least one subtype

Specialization

In a specialization hierarchy, the grouping of unique attributes into a subtype entity.

Generalization

In a generalization hierarchy, the grouping of common attributes into a supertype entity.

Entity cluster

A “virtual” entity type used to represent multiple entities and relationships in the ERD. An entity cluster is formed by combining multiple interrelated entities into a single abstract entity object. An entity cluster is considered “virtual” or “abstract” because it is not actually an entity in the final ERD

Natural key (natural identifier)

A generally accepted identifier for real-world objects

Surrogate key

A system-assigned primary key, generally numeric and autoincremented.