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Entity-Relationship Data Model. Alex Ostrovsky. Presentation Overview. Short historical overview Elements of E-R Model Basic organization & relationships in E-R Model Design principles. History of E-R Model.

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Entity-Relationship Data Model

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Entity relationship data model l.jpg

Entity-Relationship Data Model

Alex Ostrovsky


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Presentation Overview

  • Short historical overview

  • Elements of E-R Model

  • Basic organization & relationships in E-R Model

  • Design principles


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History of E-R Model

  • E-R Model was proposed by Dr. Peter Chen (currently professor at Louisiana State University)

  • Chen’s original paper on E-R Model is the 35th most sited paper in computer science

  • Chen has written papers interconnecting E-R model and linguistics


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Introduction

  • Database Structure is often called Database Schema

  • E-R model is graphical in nature, thus making it easy to analyze and observe relationship between data elements

  • Most DBMS are based upon E-R model

  • E-R model is not a good match for the sophisticated data structures required in today’s DBMS


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Elements of E-R Model

  • Data represented graphically via entity-relationship diagram which contains 3 main element types:

    • Entity sets

    • Attributes

    • Relationships


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Entity sets, Attributes, Relationships

  • Entity set

    • Is an abstract object, collection of such objects forms an entity set.

    • Similar notion as in OO design

  • Attribute:

    • Some concrete data (or object type) by which entity set is defined

  • Relationship

    • Specific connection between 2 or more entity sets


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E-R Diagram

  • Represents E-R elements by nodes of specific shape to indicate kind

    • Entity sets are represented by rectangles

    • Attributes are shown as ovals

    • Relationships correspond to diamonds

  • Simple example from the book:


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Simple illustration


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Instances of E-R diagram

  • DB described by E-R will contain specific data (i.e. database instance)

  • Each entity set will contain a particular finite set of entities

  • Each entity contains a particular value for each attribute

  • E-R data is not stored directly in DB


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E-R Relationships

  • Suppose R is a relationship connecting entity sets E and F. Then:

    • If each member of E can be connected by R to at most one member of F, then we say that R is many-one from E to F. Note that in a many-one relationship from E to F, each entity in F can be connected to many members of E.

    • If R is both many-one from E to F and many-one from F to E, then we say that R is one-one.In a one-one relationship an entity of either entity set can be connected to at most one entity of the other set.

    • If R is neither many-one from E to F or from F to E, then we say R is many-many


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Multi-way relationships

  • There is a relationship Sequel-of between the entity set Movies and itself.

  • To differentiate the two movies in a relationship, one line is labeled by the role Original and one by the role Sequel, indicating the original movie and its sequel, respectively.


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Relationships Continued

  • Some data models limit relationships to be binary

  • It is possible to convert multi-way relationship into a collection of binary many-one relationships

  • Need to introduce a connecting entity set, which will act as a bridge between smaller sets which come from splitting a larger multi-way relationship set.

  • Connecting entity set might have its own attributes


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Design Principles

  • Faithfulness:

    • Design has to comply strictly with specifications

    • Logical attributes and relationships

  • Avoid redundancy

  • "Everything should be made as simple as possible, but not simpler.” Albert Einstein

  • Choose right relationships

  • Select right elements

    • Many choices exist between using attributes and using entity set/relationship combinations

    • An attribute is simpler to implement than either entity set or a relationship


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Design Principals Cont.

  • To replace an entity set by an attribute or attributes of several entity sets 3 conditions must be enforced:

    • All relationships in which entity set is involved must have arrows entering it. That is, it must be the “one” in many-one relationships, or its generalization for the case of multi-way relationships.

    • The attributes for E must collectively identify an entity. if there are several attributes, then no attribute must depend on the other attributes

    • No relationship involves E more than once


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Thank You.


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References

  • Dr. Chen’s homepage: http://bit.csc.lsu.edu/~chen/chen.html

  • Database Systems: A First Course, J.D. Ullman & J. Widom

  • http://en.wikipedia.org/wiki/Entity-relationship_diagram

  • http://en.wikipedia.org/wiki/IDEF

  • http://en.wikipedia.org/wiki/Entity_relationship_diagram


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