Entity Relationship Diagram

What it is
Entity Relationship Diagramming is used to create a data model for the business or system.  A Data Model is a representation of the information in the business. The Entity-Relationship (ER) model was originally proposed by Peter P. Chen in 1976. Subsequently, there are many versions and notations of Data Models that are beyond the scope of this guide.

When it's Used
It can be very helpful to prepare and use a Data Model during various phases of the SDLC.  Initially, a version of the ERD may be used to elicit and document the client user requirements as a Conceptual Data Model. The ERD may be then refined to produce a Logical Data Model by data modelers to complete it and eliminate anomalies and redundancies (see Normalization). Next the Physical Data Model can be generated for the technical database design using an ERD notation.

In addition to being a communication tool for client-driven analysis, the progressive development of the data model can ensure that you are building the model effectively and addressing all of the relationships that are necessary for the business.

Description


Entity (aka Object)
 A person, place or thing. An entity represents a whole set of occurrences of the entity. 
Symbol: Rectangular box (Chen or Bachman notation)

Relationship 
The association that one entity or object has with another such as contained in, placed by, shipped to, etc.
Symbol: Line connecting the Entities (Bachman notation), Line with a Diamond in the middle (Chen notation). Labeled with the relationship verb phrase. Cardinality
The ratio of occurrences of related entities.
Symbols: 0,1,N; „  ;

Description