Entity Relationship Diagram | ER Model
In a database management system (DBMS), a group of information or data which is of interest to an organization that is called an Entity. A model that represents system data by entity and relationship sets that is called an Entity Relationship Diagram (E-R Diagram) or Entity Relationship Model (E-R Model).
The E-R model is based on a perception of real world which consists of a set of basic objects. This E-R diagram was initially developed by P.P. Chen.
Components of E-R Model : E-R Model consist of the following major components :
1. Entity : An entity is a class of persons, place, objects, events or concepts about which we need to collect and store data. Here showing different entities include :
Entity Set : An entity set is the collection of entities of the same type entities which share common properties or attributes.
Example : The set of all employees of an organization can be called as the entity set Employee.
2. Attributes : Each entity can have a number of characteristics. The characteristics of an entity are called Attributes. An attribute is a descriptive property or characteristic of an entity. Some attributes can be logically grouped into super attributes called Compound Attribute.
3. Key attributes : The Key attributes is an attribute that uniquely identifies an entity in the entity set.
4. Relationships : An association of several entities in an Entity Relation model is called Relationship.
There are three types of relationships are exist :
(i) One to One Relationship (1:1)
(ii) One to Many Relationship (1:M)
(iii) Many to Many Relationship (M:M)
(i) One to One Relationship (1:1) : (i) One to One Relationship is an association only between two entities.
Example : In a College, each department has only one head of the department.
(ii) One to Many Relationship (1:M) : One to Many Relationship exists when one entity is related to more than one entity.
Example : A father may have many children but a child has one father.
(iii) Many to Many Relationship (M:M) : A Many to Many Relationship (M:M) indicates that entities may have many relationships among each other.
Example : One customer may buy many item of goods and one item of goods may be bought by many customers.