Entity Relationship Model (Diagram)

An entity-relationship diagram is a data modeling technique that creates a graphical representation of the entities, and the relationships between entities, within an information system.


The three main components of an ERD are:

1. One-to-One
One instance of an entity (A) is associated with one other instance of another entity (B). For example, in a database of employees, each employee name (A) is associated with only one social security number (B).


2. One-to-Many
One instance of an entity (A) is associated with zero, one or many instances of another entity (B), but for one instance of entity B there is only one instance of entity A. For example, for a company with all employees working in one building, the building name (A) is associated with many different employees (B), but those employees all share the same singular association with entity A.


3. Many-to-Many
One instance of an entity (A) is associated with one, zero or many instances of another entity (B), and one instance of entity B is associated with one, zero or many instances of entity A. For example, for a company in which all of its employees work on multiple projects, each instance of an employee (A) is associated with many instances of a project (B), and at the same time, each instance of a project (B) has multiple employees (A) associated with it.




ERD-Visio Reference Site


http://msdn.microsoft.com/en-us/library/office/aa140264(v=office.10).aspx

http://techpanacea.blogspot.in/2008/07/how-to-reverse-engineer-er-diagram-from.html