Fall08-Lec2

# Fall08-Lec2 - Entity-Relationship and Enhanced...

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Sunday, September 1 Entity-Relationship and Enhanced Entity-Relationship Conceptual Data Models (Chapters 6 & Section 7.1)

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Sunday, September 2 E-R MODEL E-R model is based on a real world that consists of entities and relationships. It consists of two primitives: 1. Entities : objects that exist and are distinguishable from one another. These objects might be abstract, e.g., SF-49ers, Joe, Bob, etc. 2. Relationships : two or more entities may participate in a relationship, e.g., Joe is a fan of SF-49ers, etc. An entity is represented by a set of attributes, e.g., Joe might have a social- security number, a phone number, a spouse, and a salary An attribute is a function that maps an entity into a domain, e.g., Joe's social- security number maps Joe into the domain of integers. An attribute is represented as an A set of entities of the same type is termed an entity set , e.g., football teams. Entity sets do not need to be disjoint, e.g., students, employees, people. An entity set is represented as a
Sunday, September 3 RELATIONSHIPS Relationships of the same type are contained in a relationship set. More formally, if E 1 , E 2 , . .., E n are entity sets, then a relationship set R is a subset of: {( 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. A relationship set is represented as a A relationship may involve two entities, binary relationship: A relationship may involve three entities, ternary relationship: Football Teams fans People Football Teams fans People Date

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Sunday, September 4 A relationship does not require participation of entities from a set. In the previous example, John as an entity in “People” entity-set and is not required to participate in a relationship with “Football Teams” entity-set. There are times, when one may want to specify that all members of an entity set MUST participate in a relationship. Use double lines to represent this case. The double lines specifies total participation of entities in People entity set in the “fans” relationship set. RELATIONSHIPS Football Teams fans People
Sunday, September 5 RELATIONSHIPS (Cont…) A relationship may involve n entities, N-ary relationship It is always possible to replace a non-binary relationship set by a number of distinct binary relationship sets Football Teams fans People Date on

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Sunday, September 6 RELATIONSHIPS (Cont…) Example: A library database contains a listing of authors that have written books on various subjects (one author per book). It also contains information about libraries that carry books on various subjects.
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## This note was uploaded on 01/11/2009 for the course CSCI 485 taught by Professor Ghandeharizadeh during the Fall '08 term at USC.

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Fall08-Lec2 - Entity-Relationship and Enhanced...

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