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