Basic Concepts of Entity-Relationship (E-R) Model

Basic Concepts of Entity – Relationship (E – R) Model:

1.    Entity:

 

An entity can be defined as a ‘thing’ or ‘object’ in the real world that can be distinguished from all other objects. For ex: each person in an organization or enterprise is an entity.

 

An entity has a set of properties. Furthermore, the values for some set of properties may uniquely identify an entity. For ex: a person may have a Person-ID property, whose value uniquely identifies that person. So, the value 123-456-789 for a Person- ID would uniquely identify one particular person in an organization.

 

An entity may be concrete such as a person or a book or it may be abstract such as a loan or a holiday or a concept.

 

 

2.     Entity set:

 

An entity set can be defined as a set of entities of the same type that share (have) the same properties or attributes. For ex: the set of all persons who are customers at a given bank can be defined as the entity set Customer.

 

The individual entities that constitute a set are said to be the extension of the entity set. As such, all the individual bank customers are considered as an extension of the entity set Customer.

 

Entity sets do not need to be disjoint. For ex: it is possible to define an entity set of all employees of a bank (Employee) and the entity set of all customers of the bank (Customer). A person entity may be an employee entity, a customer entity, both or neither.

 

 

3.    Relationship:

 

A relationship can be defined as an association among several entities. For ex: we can define a relationship that associates customer Vinod with loan L-10. This relationship specifies that Vinod is a customer with a loan number L-10.

 

 

4.     Relationship set:

 

A relationship set is a set of relationships of the same type. Formally, it is a mathematical relation on n>=2 (possibly non distinct entity sets).