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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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Presentation Transcript
presentation overview
Presentation Overview
  • Short historical overview
  • Elements of E-R Model
  • Basic organization & relationships in E-R Model
  • Design principles
history of e r model
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
introduction
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
elements of e r model
Elements of E-R Model
  • Data represented graphically via entity-relationship diagram which contains 3 main element types:
    • Entity sets
    • Attributes
    • Relationships
entity sets attributes relationships
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
e r diagram
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:
instances of e r diagram
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
e r relationships
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
multi way relationships
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.
relationships continued
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
design principles
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
design principals cont
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
references
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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