E-R DIAGRAM

 ENTITY-RELATIONSHIP DIAGRAM

The other tool for problem analysis is the Entity-Relationship Diagram, often called as ER Diagram. In other words, a model that represents system data by entity and relationship sets is called E-R (Entity-Relationship) diagram. It is a detailed logical representation of the data for an organization. The entity-relationship (E-R) diagram is based on a perception of a real world which consists of a set of basic objects.

The ERD is graphical data modeling tool. It is based on a perception of a real worker that consists of a collection of basic objects , called entities , and of relation among these objects. While DFD captures the data and its relationship.

E-R diagram is consists of three main constructs, i.e. Data Entities, Relationships and their Associated Attributes.

Entities: - An entity is a person, place, thing or event of interest to the organization and about which data are captured, stored or processed. For example, an employee is an entity.

Relationship: - An association of several entities in an entity-relationship diagram is called relationship. There are three types of relationship exist among entities. These are:

One-to-One

One-to-Many

Many-to-Many

   

Attributes: - Each entity type has a set of attributes associates with it. An attribute is a property of characteristics of an entity that is of interest to the organization. I use an initial Capital Letters, followed by Lowercase Letters in naming an attribute. If an entity is something about which I want to store data, then I need to identify what specific pieces of data I want to store about each instance of a given entity. I call these pieces as data attributes.

ERD is built up by the following components :

Rectangle         which represent entity sets

Ellipses             which represent attributes

Diamonds    which represent  relationship among entity sets

Lines           which like attributes to entity sets customer set to    

                    relationship