ch7 E-R Model - Chapter 7 Entity-Relationship Model Chapter...

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Chapter 7: Entity-Relationship Model
Chapter 6: Entity-Relationship ModelDesign ProcessModelingConstraintsE-R Diagram Design Issues Weak Entity Sets Extended E-R FeaturesDesign of the Bank DatabaseReduction to Relation Schemas
Design PhasesInitial: Fully characterize the data needs of the prospective database usersDesigner chooses a data modelConceptual design (E-R Model)Specification of functional requirements.User describes the kind of operations (or transactions) that will be performed on the data (modify, update,…)Moving from abstract to implementationLogical design phasePhysical design phase (form of file organization and internal structure)Design alternatives, avoid:Redundancyincompletness
The Entity-Relationship ModelSpecification of an enterprise schema that represents the overall logical structure of a databaseUseful in mapping the meanings and interactions of real world interaction into a conceptual schemaThree basic notationsEntity sets.Entity is a thing or object in real world that is distinguishable from other objectsExample: specific person, company, event, plantEntity set is a set of entities of the same type (share same properties)Represented by a set of attributes. Do not have to be disjoint (person : employee or a customer)Example: people have names and addressesRelationship setsAssociations among several entitiesAttributes
Entity Sets instructor and studentinstructor_ID instructor_name student-ID student_name
Relationship SetsA relationshipis an association among several entitiesExample:44553 (Peltier)advisor22222 (Einstein)studententityrelationship setinstructorentityA relationship setis a mathematical relation among n2 entities, each taken from entity sets{(e1, e2, … en) | e1E1, e2E2, …, enEn}where (e1, e2, …, en) is a relationshipExample: (44553,22222) advisor
Relationship Set advisor
Relationship Sets (Cont.)An attributecan 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
Degree of a Relationship SetRefers to number of entity sets that participate in a relationship set.binary relationshipinvolve 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: studentswork on research projectsunder the guidance of an instructor. relationship proj_guideis a ternary relationship between instructor, student, and project
AttributesAn entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set.

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