Functional Dependency

Functional Dependency (FD) is a fundamental concept in relational databases used to describe the relationship between attributes (columns) of a relation (table). It helps identify how data is connected and plays a crucial role in normalization, which is the process of organizing data to reduce redundancy.

What is a Functional Dependency?

In simple terms:

If you know the value of one attribute (or a set of attributes), and you can uniquely determine the value of another attribute, then there is a functional dependency.

We write this as:

This means:

• Attribute A functionally determines attribute B.
• For every value of A, there is exactly one corresponding value of B.

Example:

Consider this table:

• StudentID → Name (StudentID uniquely determines Name)
• StudentID → Department (StudentID uniquely determines Department)

So, StudentID is the determinant, and Name and Department are dependent attributes.

But:

• Department → Name is not a valid FD, because multiple students can be in the same department.

Types of Functional Dependencies:

Trivial FD

• If B is a subset of A:
  A → B is trivial.
  Example: {StudentID, Name} → Name

Non-trivial FD

• If B is not a subset of A.
  Example: StudentID → Name

Fully Functional Dependency

• B depends on the entire set of A, not just a part.
  Used in 2NF.
  Example: {RollNo, Course} → Grade
  But not just RollNo → Grade

Transitive Dependency

• A → B and B → C, then A → C
  Used to eliminate 3NF issues.
  Example:
  StudentID → DepartmentID,
  DepartmentID → DepartmentName,
  So, StudentID → DepartmentName (transitive)

Why Functional Dependency Matters

Functional dependencies help us identify candidate keys.

They are the foundation for normalization, which ensures that:

• Data is organized efficiently.
• Redundancy is minimized.
• Update anomalies are reduced.