lec02 - The Entity-Relationship Model Chapter 2 1/6/11 EECS...

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1 1/6/11 EECS 484: Database Management Systems, Kristen LeFevre 1 The Entity-Relationship Model Chapter 2 1/6/11 EECS 484: Database Management Systems, Kristen LeFevre 2 Entity-Relationship Model Tool for describing the data involved in a real-world enterprise Helps us to go from imprecise / ambiguous “business requirements” to precise technical descriptions DBMS can understand ER Design Requirements Relational Schema Increasing Technical Precision
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2 1/6/11 EECS 484: Database Management Systems, Kristen LeFevre 3 ER Model Basics Entity: Distinguishable real-world object Described by a set of attributes ; Each attribute has a domain Entity Set : A collection of similar entities. E.g., all citizens. All entities in an entity set have the same set of attributes. (Until we consider ISA hierarchies!) Key : minimal set of attributes whose values uniquely identify an entity in an entity set 1 Primary key Multiple Candidate keys Pictorially … Citizen ssn name bday 1/6/11 EECS 484: Database Management Systems, Kristen LeFevre 4 ER Model Basics Relationship : Association among two or more entities Relationship Set : Collection of similar relationships Citizen ssn name bday PR-Cand cid waddr budget Vote when Poll Stn pid addr size Citizen ssn name bday Represents Elected official Constituents Descriptive Attribute An entity set can participate in Relationship set more than once, in different roles
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3 1/6/11 EECS 484: Database Management Systems, Kristen LeFevre 5 Key Constraints Key Constraint : Each citizen votes at most once 1-to-1 1-to Many Many-to-1 Many-to-Many Citizens Pr-Cands c2 c3 p2 p3 Citizen ssn name bday PR-Cand cid waddr budget PR-vote when 1/6/11 EECS 484: Database Management Systems, Kristen LeFevre 6
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This note was uploaded on 12/08/2011 for the course EECS 484 taught by Professor Staff during the Winter '08 term at University of Michigan.

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lec02 - The Entity-Relationship Model Chapter 2 1/6/11 EECS...

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