02010_ch06

02010_ch06 - Database Systems: Design, Implementation, and...

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Database Systems: Design, Implementation, and Management Eighth Edition Chapter 6 Advanced Data Modeling
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th 2 Objectives In this chapter, you will learn: About the extended entity relationship (EER) model’s main constructs How entity clusters are used to represent multiple entities and relationships The characteristics of good primary keys and how to select them How to use flexible solutions for special data modeling cases What issues to check for when developing data models based on EER diagrams
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th 3 The Extended Entity Relationship Model Result of adding more semantic constructs to original entity relationship (ER) model Diagram using this model is called an EER diagram (EERD)
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th 4 Entity Supertypes and Subtypes Entity supertype Generic entity type related to one or more entity subtypes Contains common characteristics Entity subtypes Contains unique characteristics of each entity subtype
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th 5
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th 6 Specialization Hierarchy Depicts arrangement of higher-level entity supertypes and lower-level entity subtypes Relationships described in terms of “IS-A” relationships Subtype exists only within context of supertype Every subtype has only one supertype to which it is directly related Can have many levels of supertype/subtype relationships
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th 7
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th 8 Inheritance Enables entity subtype to inherit attributes and relationships of supertype All entity subtypes inherit their primary key attribute from their supertype At implementation level, supertype and its subtype(s) maintain a 1:1 relationship Entity subtypes inherit all relationships in which supertype entity participates Lower-level subtypes inherit all attributes and relationships from all upper level-supertypes
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th 9
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th 10 Subtype Discriminator Attribute in supertype entity Determines to which entity subtype each supertype occurrence is related Default comparison condition for subtype discriminator attribute is equality comparison Subtype discriminator may be based on other comparison condition
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th 11 Disjoint and Overlapping Constraints Disjoint subtypes Also known as non-overlapping subtypes Subtypes that contain unique subset of supertype entity set Overlapping subtypes Subtypes that contain nonunique subsets of supertype entity set
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th 12
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th 13
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th 14 Completeness Constraint Specifies whether entity supertype occurrence must be a member of at least one subtype Partial completeness Symbolized by a circle over a single line Some supertype occurrences that are not members of any subtype Total completeness Symbolized by a circle over a double line Every supertype occurrence must be member of at least one subtype
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th 15
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02010_ch06 - Database Systems: Design, Implementation, and...

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