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