Entity-relationship model
In software engineering, an entity–relationship model (ER model) is a data model for describing the data or information aspects of a business domain or its process requirements, in an abstract way that lends itself to ultimately being implemented in a database such as a relational database.
The main components of ER models are entities (things) and the relationships that can exist among them.
Entity–relationship modeling was developed by Peter Chen and published in a 1976 paper. However, variants of the idea existed previously, and have been devised subsequently such as supertype and subtype data entities and commonality relationships.
Introduction
An entity–relationship model is a systematic way of describing and defining a business process.
The process is modeled as components (entities) that are linked with each other by relationships that express the dependencies and requirements between them.
For example, one building may be divided into zero or more apartments, but one apartment can only be located in one building.
Entities may have various properties (attributes) that characterize them.
Diagrams created to represent these entities, attributes, and relationships graphically are called entity–relationship diagrams.
An ER model is typically implemented as a database. In the case of a relational database, which stores data in tables, every row of each table represents one instance of an entity. Some data fields in these tables point to indexes in other tables; such pointers represent the relationships.
The three schema approach to software engineering uses three levels of ER models that may be developed.
