### E-R Diagram

ER-Diagram is a visual representation of data that describes how data is related to each other.

### Components of E-R Diagram

The E-R diagram has three main components.

#### 1) Entity

An Entity can be any object, place, person or class. In E-R Diagram, an entity is represented using rectangles. Consider an example of an Organisation. Employee, Manager, Department, Product and many more can be taken as entities from an Organisation.

#### Weak Entity

Weak entity is an entity that depends on another entity. Weak entity doen't have key attribute of their own. Double rectangle represents weak entity.

#### 2) Attribute

An Attribute describes a property or characterstic of an entity. For example, Name, Age, Address etc can be attributes of a Student. An attribute is represented using eclipse.

#### Key Attribute

Key attribute represents the main characterstic of an Entity. It is used to represent Primary key. Ellipse with underlying lines represent Key Attribute.

#### Composite Attribute

An attribute can also have their own attributes. These attributes are known as Composite attribute.

#### 3) Relationship

A Relationship describes relations between entities. Relationship is represented using diamonds.

There are three types of relationship that exist between Entities.

• Binary Relationship
• Recursive Relationship
• Ternary Relationship

#### Binary Relationship

Binary Relationship means relation between two Entities. This is further divided into three types.

1. One to One : This type of relationship is rarely seen in real world.
2. The above example describes that one student can enroll only for one course and a course will also have only one Student. This is not what you will usually see in relationship.

3. One to Many : It reflects business rule that one entity is associated with many number of same entity. For example, Student enrolls for only one Course but a Course can have many Students.
4. The arrows in the diagram describes that one student can enroll for only one course.

5. Many to Many :
6. The above diagram represents that many students can enroll for more than one courses.

#### Recursive Relationship

When an Entity is related with itself it is known as Recursive Relationship.

#### Ternary Relationship

Relationship of degree three is called Ternary relationship.