What is Functional Dependency?

In Database Management System, a Functional Dependency (FD) defines a relationship between two attributes.

If one attribute uniquely determines another, it is called a functional dependency.

Written as:

Meaning:

• If you know X, you can uniquely determine Y

Tabular Example

Table: Student

Functional Dependencies:

Because each Roll_No is unique.

Technical Example

Let:

This means:

• For any two rows:
  • If A₁ = A₂
  • Then B₁ must equal B₂

No ambiguity allowed.

Types of Functional Dependencies

1. Trivial FD

Always true

2. Non-Trivial FD

When B is not part of A

3. Completely Non-Trivial FD

No overlap between A and B

1.College Roll Number System

Imagine your college attendance system.

• Each student has a unique Roll Number
• That Roll Number gives:
  • Name
  • Course
  • Section

So:

Why it works:
No two students share the same Roll Number.

But:

Because multiple students can have the same name.

2. Bank Account System

Think about your bank.

• Each account has a unique Account Number
• From that, you get:
  • Account Holder Name
  • Balance
  • Branch

So:

Real understanding:
Even if two people have the same name, account numbers are always unique.

3. Employee ID in Company

Inside a company:

• Every employee has a unique Employee ID
• That determines:
  • Salary
  • Department
  • Role

So:

Insight:
Two employees can have the same role but never the same ID.

4. Vehicle Registration System

Your vehicle number plate:

• Is unique
• Gives:
  • Owner Name
  • Vehicle Type
  • Registration Details

So:

Important:
The owner's name alone cannot identify the vehicle.