Lecture_6_-_Advanced_Data_Modeling

Lecture_6_-_Advanced_Data_Modeling - CSCI-507 Database...

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1 CSCI-507 Database Theory Lecture 6 Advanced Data Modeling 2 Objectives • In this lecture, students will learn: – About the extended entity relationship (EER) model – 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
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2 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) 4 Entity Supertypes and Subtypes Entity supertype – Generic entity type related to one or more entity subtypes – Contains common characteristics Entity subtype – Contains unique characteristics of each entity subtype
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3 5 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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4 7 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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5 9 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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6 11 Disjoint and Overlapping Constraints Disjoint subtypes – Also called nonoverlapping subtypes – Subtypes that contain unique subset of supertype entity set Overlapping subtypes – Subtypes that contain nonunique subsets of supertype entity set 12
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7 13 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 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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This note was uploaded on 04/15/2011 for the course CSCI 507 taught by Professor Rashid during the Spring '11 term at George Mason.

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Lecture_6_-_Advanced_Data_Modeling - CSCI-507 Database...

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