Entity Relationship Model (ER)

Entity- An entity can be a real world object that can be easily identified. It is a piece of concept about which data is stored.

 For example- In a school, students, teachers, classes, and courses considered as entities. All these entities have some attributes or properties that give them their identity.

Relationships between Entities

A Relationship is association among entities. There are three types of relationships between entities

1. One to one
2. One to many
3. Many to many

One to One relationship- One entity from entity set A will be related to at the most one entity of entity set B and contrariwise (vice-versa) of it.

 For example- There is only one department head in a department so it is one to one relationship.

One to Many relationship − More than one entities from entity set A will be related to at the most one entity of entity set B, however an entity from  entity set B can be related with more than one entity from entity set A.

 For example- There is only one class teacher among students so this shows many to one relationship.

 Many to Many relationship − One entity from A can be associated with more than one entity from B and vice versa of it.

  For example- Many courses having many students, many students doing many courses.