Entity Relationship Modeling

Entity Relationship Modeling or ER modeling is an easy and efficient technique to understand the entities (data elements) of a system and their relationship. It is a graphical technique to organize the data before you decide its database implementation.

Terms used in Entity Relationship Modeling


An entity is always considered in the scenario for which we are going to collect data. Anything that exists independently and must have data to be stored about is called an entity. In a book store a book is an entity.

Entity instance

A specific instance of an entity type is called an entity instance. In a book store “Fundamentals of Database Systems” is an Entity Instance


All the features, properties or characteristics defining an entity type are called attributes. In the bookstore example books have attributes like Title, Author, Publisher, Edition, Pages, Price and cover type.


Entities have associations among each other. These associations are added to the ER models as relationships of various kinds. In book store example entities customer buy the entity book. They have the “bought by” or “sold to” relationship.


In Entity Relationship Modeling, t is the set of values used to assign to the attributes of the entities. In the example the attribute cover type has domain {Hard Cover, paperback}.

Key Attribute

Among the assigned or selected attribute, an attribute or a combination of attributes is unique for every entity instance. This attribute is called key attribute. If it consists of more than one attributes it is called a composite key. Book Title + Author name is the composite key in our example.

Attribute Types

A Simple attribute is defined as an attribute which cannot be broken down into simpler attributes. Price of the book is a simple attribute.

A Composite attribute is one that can be divided into individual simple attributes. Author Name can be divided into First Name, Last Name and Middle Name.

A Single-Valued attribute can be assigned single value for one entity instance. In the example one book can have only one price or page count. These are single valued attributes.

In Entity Relationship Modeling, a Multi-valued attribute can have more than one value for a single entity instance. A book can have more than one author.