Presentation on theme: "THE EXTENDED ENTITY RELATIONSHIP MODEL (EERM)"— Presentation transcript:
1 THE EXTENDED ENTITY RELATIONSHIP MODEL (EERM) CHAPTER 6.1THE EXTENDEDENTITY 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.
2 Entity Supertypes and Subtypes Specialization hierarchyDepicts the arrangement of higher-level entity supertypes (parent entities) and lower-level entity subtypes (child entities)Supertype entityContains shared attributesSubtype entityContains unique attributesSupertype and its subtype(s) maintain a 1:1 relationship.
3 Disjoint Subtypes Also known as non-overlapping subtypes. Subtypes that contain a subset of the supertype entity setEach entity instance (row) of the supertype can appear in ONLY ONE of the disjoint subtypesDisjoint subtypes are indicated by the symbol “G” (Chen) or “d” (Crow Foot).
4 Overlapping SubtypesOverlapping 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).
6 A Specialization Hierarchy with Overlapping Subtypes
7 COMPLETENESS CONSTRAINTS Specifies whether each entity occurrence must also be a member of subtypePartial completenessSome supertype occurrences are NOT members of any subtypeSingle horizontal lineComplete completenessMust be a member of AT LEAST ONE subtypeDouble horizontal lineRefer Pg. 185 & 188