The E-R Model

 

Basic ER Modeling Concepts

Entity - a class of real world objects having common characteristics and properties about which we wish to record information.

Relationship - an association among two or more entities

* occurrence – instance of a relationship is the collective instances of the related entities

* degree – number of entities associated in the relationship (binary, ternary, other n-ary)

* connectivity – one-to-one, one-to-many, many-to-many

* existence dependency (constraint) – optional/mandatory

 

Attribute - a characteristic of an entity or relationship

* Identifier – uniquely determines an instance of an entity

* Identity dependence – when a portion of an identifier is inherited from another entity

* Multi-valued – same attribute having many values for one entity

* Surrogate – system created and controlled unique key (e.g. Oracle’s “create sequence”)

 

 Symbols used in ER Diagrams

Symbols in ER Diagrams

Symbols in ER Diagrams 1Symbols in ER Diagrams 2

Symbols in ER Diagrams 3

Data models

  • l  A collection of tools for describing
  1. –         Data
  2. –         Data relationships
  3. –         Data semantics
  4. –         Data constraints
  • l  Relational model
  • l  Entity-Relationship data model (mainly for database design)
  • l  Object-based data models (Object-oriented and Object-relational)
  • l  Semistructured data model  (XML)
  • l  Other older models:
  1. –         Network model
    –         Hierarchical model

 

 

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