Entity Relationship Model
Entity Relationship Model: The Entity-Relationship Model (or E-R Model) is a popular high conceptual data model. Peter Chen introduced this model in 1976 and is widely used for local design. The E-R model is used to construct the conceptual data model, which is the representation of the structure of the database that is independent of the DBMS software. The entity relationship model is mostly commonly used during the analysis phase of the database. The entity relationship model is expressed in terms of entities, relationships between the entities and the attribute or the properties of the involved entities. The entity relationship model also allows the logical designer to specify constraints on the entities or relationships between entities.
ER Diagrams
One of the most useful and attractive feature of E-R model is that it provides a graphical method for representing the logical structure of the database. For this purpose E-R Diagram are used. It means that E-R Model is expressed as an E-R diagram, which is graphical representation of an Entity Relation model. Entity Relation Model is one of the common type of model represents the entities of database through different diagrams. These diagram plays an important role in entity relationship model.
The Entity-Relationship Diagram (ERD) represents the relationship of entities graphically. The type of relationship between the entities is also represented in the E-R diagram. Basically, E-R diagrams are the design tools and documentation for the system.
Different symbols are used in E-R Diagram
The rectangular box is used to represent the entity
The double line rectangular box is used to represent weak entity.
The rectangular box with rounded edges is used to represent IS-A relation in entity relationship model.
A simple attribute is represented by placing its name in ellipse shape in entity relationship model.
A multi-valued attribute is indicted by double line ellipse.
A relationship between entities is indicated by a diamond shape.
