Attributes

Attributes are the properties or characteristics of an entity in a database. They represent the data stored about the entity and are implemented as columns in a table. Graphically represented as ovals or circles.

Here are examples of different types of attributes with explanations,

1. Simple Attribute

• Definition: Cannot be divided into smaller parts.

• Example:

  • Entity: Student

  • Attributes: StudentID, Name, Age

  • Explanation: StudentID is a simple, indivisible identifier for a student.

Table Example

CREATE TABLE Student (
    StudentID SERIAL PRIMARY KEY,
    Name TEXT NOT NULL,
    Age INT
);

2. Composite Attribute

• Definition: Can be divided into smaller sub-parts, each with independent meaning.

• Example:

  • Entity: Student

  • Attribute: FullName (composed of FirstName and LastName)

  • Explanation: FullName can be divided into FirstName and LastName.

Table Example

CREATE TABLE Student (
    StudentID SERIAL PRIMARY KEY,
    FirstName TEXT NOT NULL,
    LastName TEXT NOT NULL
);

3. Derived Attribute

• Definition: Can be calculated from other attributes.

• Example:

  • Entity: Employee

  • Attributes: DateOfBirth, Age

  • Explanation: Age is derived from DateOfBirth.

Table Example

CREATE TABLE Employee (
    EmployeeID SERIAL PRIMARY KEY,
    Name TEXT NOT NULL,
    DateOfBirth DATE
);
-- Age can be calculated as:
-- SELECT DATE_PART('year', AGE(DateOfBirth)) AS Age FROM Employee;

4. Multivalued Attribute

• Definition: Can have more than one value for a single entity.

• Example:

  • Entity: Student

  • Attribute: PhoneNumbers (a student may have multiple phone numbers)

  • Explanation: A student can have a home phone and a mobile phone.

Table Example

To handle multivalued attributes, we use a separate table

CREATE TABLE Student (
    StudentID SERIAL PRIMARY KEY,
    Name TEXT NOT NULL
);

CREATE TABLE StudentPhone (
    StudentID INT REFERENCES Student(StudentID),
    PhoneNumber TEXT NOT NULL
);

5. Key Attribute

• Definition: Uniquely identifies an entity in a table.

• Example:

  • Entity: Student

  • Attribute: StudentID

  • Explanation: StudentID is a unique identifier for each student.

Table Example

CREATE TABLE Student (
    StudentID SERIAL PRIMARY KEY,
    Name TEXT NOT NULL
);

Summary

Type	Description	Example

Simple	Cannot be divided further.	Name, Age

Composite	Can be divided into sub-parts.	FullName → FirstName, LastName

Derived	Calculated from other attributes.	Age from DateOfBirth

Multivalued	Can have multiple values for one entity.	PhoneNumbers

Key	Uniquely identifies an entity.	StudentID

Foreign Key	References the primary key of another entity.	StudentID in Enrollment table