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

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