Entity-relationship_model

Entity-relationship_model - Entity-relationship model From...

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Entity-relationship model From Wikipedia, the free encyclopedia Jump to: navigation , search A sample Entity-relationship diagram using Chen's notation In software engineering , an entity-relationship model ( ERM ) is an abstract and conceptual representation of data . Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database , and its requirements in a top-down fashion. Diagrams created by this process are called entity-relationship diagrams , ER diagrams , or ERDs . This article refers to the techniques proposed in Peter Chen 's 1976 paper. [1] However, variants of the idea existed previously, [2] and have been devised subsequently.
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Contents 1 Overvi ew 2 The buildin g blocks: entities, relation ships, and attribut es 3 Relatio nships, roles and cardina lities 3 . 1 R e l a t i o n s h i p N a m e s 3 . 2 R o l
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[ edit ] Overview The first stage of information system design uses these models during the requirements analysis to describe information needs or the type of information that is to be stored in a database . The data modeling technique can be used to describe any ontology (i.e. an overview and classifications of used terms and their relationships) for a certain area of interest . In the case of the design of an information system that is based on a database, the conceptual data model is, at a later stage (usually called logical design), mapped to a logical data model , such as the relational model ; this in turn is mapped to a physical model during physical design. Note that sometimes, both of these phases are referred to as "physical design". A UML metamodel of Extended Entity Relationship models [ edit ] The building blocks: entities, relationships, and attributes Two related entities An entity with an attribute A relationship with an attribute Primary key An entity may be defined as a thing which is recognized as being capable of an independent existence and which can be uniquely identified. An entity is an abstraction from the complexities of some domain. When we speak of an entity we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world. [3] An entity may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a customer transaction or order. Although the term entity is the one most commonly used, following Chen we should really distinguish between an entity and an entity-type. An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type. There are usually many instances of an entity-type. Because the term entity-type is somewhat cumbersome, most people tend to use the term entity as a synonym for this term.
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Entities can be thought of as nouns . Examples: a computer, an employee, a song, a mathematical theorem. A relationship captures how two or more entities are related to one another. Relationships can be thought of
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This note was uploaded on 09/30/2011 for the course MIS 325 taught by Professor Mote during the Spring '08 term at University of Texas.

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Entity-relationship_model - Entity-relationship model From...

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