Presentation on theme: "THE EXTENDED ENTITY RELATIONSHIP MODEL (EERM)"— Presentation transcript:
1THE 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.
2Entity 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.
3Disjoint 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).
4Overlapping 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).
6A Specialization Hierarchy with Overlapping Subtypes
7COMPLETENESS 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