Attributes and Domains

Attribute

An attribute is a characteristic or property that describes an entity and provides information about it.
For example, if Employee is an entity, then Emp_ID, Name, Salary, and Department are its attributes.
Attributes help us store and identify the important details of each entity.

Domain

A domain is the set of all permitted or valid values that a particular attribute can have.
For example, the domain of an Age attribute may be positive integers between 18 and 65.
Entering a value such as “Twenty” in an integer-based Age attribute would be invalid.

Types of Attributes

1. Simple (Atomic) Attribute

A Simple Attribute is an attribute that cannot be divided further into smaller meaningful components.
It represents a single property of an entity and stores one basic piece of information.
Example: Age, Gender, Roll_No.

2. Composite Attribute

A Composite Attribute is an attribute that can be divided into smaller sub-components, where each component has its own meaning.
These smaller attributes together represent the complete value of the main attribute.
Example: Name can be divided into First_Name, Middle_Name, and Last_Name.

3. Single-Valued Attribute

A Single-Valued Attribute can have only one value for a particular entity instance.
It represents information where an entity is expected to have one value at a given time.
Example: Date_of_Birth or Aadhaar_Number for a particular person.

4. Multi-Valued Attribute

A Multi-Valued Attribute can have more than one value for a single entity instance.
It is useful when an entity can have multiple values for the same type of information.
Notation: It is represented by a Double Oval. Example: Phone_Number or Skills.

5. Derived Attribute

A Derived Attribute is an attribute whose value is calculated from another stored attribute rather than being stored directly.
Its value may change automatically when the value from which it is derived changes.
Notation: It is represented by a Dashed Oval. Example: Age can be derived from Date_of_Birth.

6. Key Attribute

A Key Attribute is an attribute that uniquely identifies each entity instance within an entity set.
No two entity instances should have the same value for the key attribute.
Notation: It is represented by an Oval with the attribute name underlined. Example: Roll_No for a student.