An entity-relationship model (ERM) is an abstract and conceptual representation of data, information aspects of a business domain or its process requirements. Ultimately ERM is being implemented in a database.
Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements in a top-down fashion. Diagrams created by this process are called entity-relationship diagrams, ER diagrams, or ERDs.
The main components of the ER diagrams are the entities and the relationships that exist among them. For example with the original notation, in the relational databases tables are shown as boxes, its attributes as ovals and their relations with other tables with lines.
The three schema approach to software engineering uses three levels of ER models that may be developed:
Conceptual data model is the highest ER model, that contain least granular detail but establishes the overall scope of what to be included in the model set.
Logical data model contains more detail than the conceptual ER model. Operational and transcational data entities are defined.
Physical data model - one or more such models can be developed from each logical ER model. Normally developed to be instantiated as a database, therefore each physical model must contain enough detail to produce a database.
