CHAPTER 6.1

THE EXTENDED ENTITY RELATIONSHIP MODEL

(EERM)

• Sometimes referred also as “enhanced entity relationship model”.

• It is the result of adding more semantic constructs to the original entity relationship (ER) model.

• To follow closely the complexity of real world scenario.

Entity Supertypes and Subtypes

• Specialization hierarchy

– Depicts the arrangement of higher-level entity supertypes (parent entities) and lower-level entity subtypes (child entities)

• Supertype entity

– Contains shared attributes

• Subtype entity

– Contains unique attributes

• Supertype and its subtype(s) maintain a 1:1 relationship.

Disjoint Subtypes

• Also known as non-overlapping subtypes.

• Subtypes that contain a subset of the supertype entity set

• Each entity instance (row) of the supertype can appear in ONLY ONE of the disjoint subtypes

• Disjoint subtypes are indicated by the symbol “G” (Chen) or “d” (Crow Foot).

Overlapping Subtypes

• Overlapping subtypes are subtypes that contain a subset of the supertype entity set.

• However, each entity occurrence (row) of the supertype can appear in MORE THAN ONE subtypes.

• Overlapping subtypes are indicated by the symbol “Gs” (Chen) or “o” (Crow Foot).

A Specialization Hierarchy

A Specialization Hierarchy with Overlapping Subtypes

COMPLETENESS CONSTRAINTS

• Specifies whether each entity occurrence must also be a member of subtype

• Partial completeness

– Some supertype occurrences are NOT members of any subtype

– Single horizontal line

• Complete completeness

– Must be a member of AT LEAST ONE subtype

– Double horizontal line

• Refer Pg. 185 & 188