Presentation on theme: "CHAPTER 6.1 THE EXTENDED ENTITY RELATIONSHIP MODEL (EERM) Sometimes referred also as enhanced entity relationship model. It is the result of adding more."— Presentation transcript:
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