What is ER Modeling?

What is ER Modeling?

Entity Relationship Modeling (ER Modeling) is a graphical approach to database design. It uses Entity/Relationship to represent real world objects.

Entity

An Entity is a thing or object in real world that is distinguishable from surrounding environment. For example each employee of an organization is a separate entity. Following are some of major characteristics of entities.

• An entity has a set of properties.
• Entity properties can  have values. 

Let’s consider our first example again. An employee of an organization is an entity. If “Peter” is a programmer (an employee) at Microsoft, he can have attributes (properties) like name, age, weight, height, etc. It is obvious that those do hold values relevant to him.

Attributes

Attribute is a properties of entity.

Each attribute can have Values. In most cases single attribute have one value. But it is possible for attributes have multiple values also.  For example Peter’s age has a single value. But his “phone numbers” property can have multiple values.

Relationships

It refers to the relation which exists between data of one entity and data of another entity.

Entities can have relationships with each other. Let’s consider a simplest example. Assume that each Microsoft Programmer is given a Computer. It is clear that that Peter’s Computer is also an entity. Peter is using that computer and the same computer is used by Peter. In other words there is a mutual relationship among Peter and his computer.

There are three types of relationships between entities:

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.