ER Model Tutorial – Concepts of Entity, Attribute, and Relationship
Are you searching for ER Model Tutorial? Here I’m describing the concepts of ER Model, Entity, Attribute, and Relationship.
ER Model Tutorial
ER Model (Entity-Relationship Model) is a conceptual data model in DBMS. The ER model is very useful in mapping the meaning and interactions of real world enterprises onto a conceptual schema.
Concepts of ER Model
There are three basic concepts in Entity-Relationship model. They are:
- Entity Sets
- Relationship Sets, and
- An entity is a thing or object in the real world that is distinguishable from all other objects.
Example: Each student in a school.
- An entity set is the set of entities which share the same properties.
Example: The collection of all students in the school. Where all of the students should have properties like admission_no, name, address and date_of_birth.
- A relationship is an association among several entities.
Example: Consider the example of student and project. Here the student work on the project. So the relationship in this relation is “work_on”.
- Attributes are the properties of entity.
Example: The entity student has the properties like admission_no, name, address and date_of_birth.
- The collection of values of an attribute is known as value set or domain.
Example: The collection of all admission numbers from the database of a school.
Categorization of Attributes
The attributes can be mainly categorized into four groups. They are described below.
Simple v/s Composite Attributes
- Simple attributes cannot further be divided into sub parts (sub attributes).
Example: The attribute phone cannot further be divided.
- Composite attributes can be further divided.
Example: The attribute name can further be divided into sub parts like first_name, middle_name, and last_name.
Single Valued v/s Multi Valued Attributes
- The attribute that can have only one single value is called Single valued attribute.
Example: The attribute date_of_birth can have only one value.
- The attribute that can have multiple values is called Multi valued attribute.
Example: The attribute phone_no can have multiple phone numbers.
Derived v/s Stored Attributes
- The Stored attributes are the attribute that have a direct value stored.
Example: The attribute date_of_birth has a direct stored value.
- The Derived attributes are the attributes that doesn’t have a direct values, but the values are derived from stored attributes.
Example: The attribute age is derived from the date_of_birth attribute.
- A particular entity may not have a applicable value for an attribute. This type of attributes are called Null Attributes.