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Entity-Relationship Data Model

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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  1. Entity-Relationship Data Model Alex Ostrovsky

  2. Presentation Overview • Short historical overview • Elements of E-R Model • Basic organization & relationships in E-R Model • Design principles

  3. 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

  4. 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

  5. Elements of E-R Model • Data represented graphically via entity-relationship diagram which contains 3 main element types: • Entity sets • Attributes • Relationships

  6. 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

  7. 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:

  8. Simple illustration

  9. 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

  10. 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

  11. 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.

  12. 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

  13. 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

  14. 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

  15. Thank You.

  16. 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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