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.