Functional Dependency

Functional dependency is a relationship that exists between two attributes. It typically exists between the primary key and non-key attribute within a table.

1. X   →   Y  

The left side of FD is known as a determinant, and the right side of the production is known as a dependent.

For example:

Assume we have an employee table with Emp_Id, Emp_Name, Emp_Addressand  attributes.

Here Emp_Id attribute can uniquely identify the Emp_Name attribute of the employee table because if we know the Emp_Id, we can tell that the employee name is associated with it.

Functional dependency can be written as:

1. Emp_Id → Emp_Name   

We can say that Emp_Name is functionally dependent on Emp_Id.

Types of Functional dependency

1. Trivial functional dependency

• A → B has trivial functional dependency if B is a subset of A.
• The following dependencies are also trivial: A → A, B → B

Example:

1. Consider a table with two columns Employee_Id and Employee_Name.  
2. {Employee_id, Employee_Name}   →    Employee_Id is a trivial functional dependency as   
3. Employee_Id is a subset of {Employee_Id, Employee_Name}.  
4. Also, Employee_Id → Employee_Id and Employee_Name   →    Employee_Name are trivial dependencies too.  

2. Non-trivial functional dependency

• A → B has a non-trivial functional dependency if B is not a subset of A.
• When A intersection B is NULL, then A → B is called a complete non-trivial.

Example:

1. ID   →    Name,  
2. Name   →    DOB