ch7_ER-Model

ch7_ER-Model - Database System Concepts, 6 th Ed ....

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Unformatted text preview: Database System Concepts, 6 th Ed . Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 7: Entity-Relationship Model Chapter 7: Entity-Relationship Model Silberschatz, Korth and Sudarshan 7.2 Database System Concepts - 6 th Edition Chapter 7: Entity-Relationship Model Chapter 7: Entity-Relationship Model Design Process Modeling Constraints E-R Diagram Design Issues Weak Entity Sets Extended E-R Features Design of the Bank Database Reduction to Relation Schemas Database Design UML Silberschatz, Korth and Sudarshan 7.3 Database System Concepts - 6 th Edition Modeling Modeling A database can be modeled as: a collection of entities, relationship among entities. An entity is an object that exists and is distinguishable from other objects. Example: specific person, company, event, plant Entities have attributes Example: people have names and addresses An entity set is a set of entities of the same type that share the same properties. Example: set of all persons, companies, trees, holidays Silberschatz, Korth and Sudarshan 7.4 Database System Concepts - 6 th Edition Entity Sets Entity Sets instructor instructor and and student student instructor_ID instructor_name student-ID student_name Silberschatz, Korth and Sudarshan 7.5 Database System Concepts - 6 th Edition Relationship Sets Relationship Sets A relationship is an association among several entities Example: 44553 (Peltier) advisor 22222 (Einstein) student entity relationship set instructor entity A relationship set is a mathematical relation among n 2 entities, each taken from entity sets {( e 1 , e 2 , e n ) | e 1 E 1 , e 2 E 2 , , e n E n } where ( e 1 , e 2 , , e n ) is a relationship Example: (44553,22222) advisor Silberschatz, Korth and Sudarshan 7.6 Database System Concepts - 6 th Edition Relationship Set Relationship Set advisor advisor Silberschatz, Korth and Sudarshan 7.7 Database System Concepts - 6 th Edition Relationship Sets (Cont.) Relationship Sets (Cont.) An attribute can also be property of a relationship set. For instance, the advisor relationship set between entity sets instructor and student may have the attribute date which tracks when the student started being associated with the advisor Silberschatz, Korth and Sudarshan 7.8 Database System Concepts - 6 th Edition Degree of a Relationship Set Degree of a Relationship Set binary relationship involve two entity sets (or degree two). most relationship sets in a database system are binary. Relationships between more than two entity sets are rare. Most relationships are binary. (More on this later.) Example: students work on research projects under the guidance of an instructor ....
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This note was uploaded on 09/10/2011 for the course CS 308 taught by Professor Frankl during the Spring '11 term at NYU Poly.

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ch7_ER-Model - Database System Concepts, 6 th Ed ....

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