1 / 14

Entity Relationship Modelling

Entity Relationship Modelling. What is Entity Relationship Modelling?. The Entity-Relationship model is – “ a data model for high-level descriptions of conceptual data models, and it provides a graphical notation for representing such data models in the form of entity-relationship diagrams. ”

aminia
Download Presentation

Entity Relationship Modelling

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Entity Relationship Modelling

  2. What is Entity Relationship Modelling? The Entity-Relationship model is – “a data model for high-level descriptions of conceptual data models, and it provides a graphical notation for representing such data models in the form of entity-relationship diagrams.” [Wikipedia Website]

  3. What does this mean? Basically a entity-relationship model is – • A simple way to representing on paper, an idea for a data model e.g. a database. • With special graphical systems and symbols.

  4. Key Terms • Entity • Relationship • Attribute • Primary Key

  5. What is an Entity? • An entity represents a discrete object. • Entities are generally nouns. Examples include – • a computer, an employee, a song.

  6. Relationships • A relationship captures how two or more entities are related to one another. • Relationships can be thought of (roughly) as verbs. Examples include – • an owns relation between a company and a computer. • a supervises relation between an employee and a department • a performs relation between an artist and a song

  7. Attributes Entities and relationships can both have attributes. Examples – • an employee entity might have a national insurance number attribute. • the proved relation may have a date attribute.

  8. Primary Keys • A primary key is an attribute which can uniquely identify an Entity. Example – A National Insurance number, as each person has a different number to everyone else.

  9. Entity Relationship Diagrams (ERDs) Important - • Entity-relationship diagrams don't show single entities or single instances of relations. • they show entity sets and relationship sets. Example – • a particular song is an entity. • The collection of all songs in a database is an entity set.

  10. Common Notation There are many systems of notation for ERDs. These are the common symbols used - • Entity Sets - drawn as rectangles. • Relationships - drawn as diamonds. • Attributes - drawn as ovals connected to their owning entity sets by a line.

  11. Less Common Notion Weak Entities • Cannot be identified by their attributes alone • Therefore must use as its primary key its own attributes and the primary key of an entity it is related to.

  12. Example Models Simple Model –

  13. Example Models (Cont.) More advanced model –

  14. Further Reading • Wikipedia Website – http://en.wikipedia.org/wiki/Entity_relationship_diagram • Peter Pin-Shan Chen - “The Entity-Relationship Model-Toward a Unified View of Data” http://bit.csc.lsu.edu/~chen/pdf/erd.pdf

More Related