Lecture 2 - ER Model - Introduction to Database Systems IS...

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Lecture 2 Entity-Relationship Model Introduction to Database Systems IS 230 This is the instructor’s notes and student has to read the textbook for complete material. Dr. Jawad Berri
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J. Berri Entity-Relationship Model Slide 2 Chapter 2: Entity-Relationship Model 1. Entity Sets 2. Relationship Sets 3. Mapping Cardinalities 4. Constraints 5. Keys 6. Design Issues 7. Weak Entity Sets 8. Extended E-R Features 9. Design of an E-R Database Schema
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J. Berri Entity-Relationship Model Slide 3 1. Entity Sets 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
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J. Berri Entity-Relationship Model Slide 4 Entity Sets customer and loan customer-id   customer-  customer-  customer-           loan-    amount                           name     street         city                    number
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J. Berri Entity-Relationship Model Slide 5 Attributes An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set. Example: customer = (customer-id, customer-name, customer-street, customer-city) loan = (loan-number, amount) Domain – the set of permitted values for each attribute Attribute types: Simple and composite attributes. Single-valued and multi-valued attributes E.g. multivalued attribute: phone-numbers Derived attributes Can be computed from other attributes E.g. age , given date of birth
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J. Berri Entity-Relationship Model Slide 6 Composite Attributes
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J. Berri Entity-Relationship Model Slide 7 2. Relationship Sets A relationship is an association among several entities Example: Hayes depositor A-102 customer entityrelationship set account 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: (Hayes, A-102) depositor
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J. Berri Entity-Relationship Model Slide 8 Relationship Set borrower
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J. Berri Entity-Relationship Model Slide 9 Relationship Sets (Cont.) An attribute can also be property of a relationship set. For instance, the depositor relationship set between entity sets customer and account may have the attribute access-date
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J. Berri Entity-Relationship Model Slide 10 2.1. Degree of a Relationship Set Refers to number of entity sets that participate in a relationship set.
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