Functional Dependency is a relationship that exists between multiple attributes of a relation.
This concept is given by E. F. Codd.
Functional dependency represents a formalism on the infrastructure of relation.
It is a type of constraint existing between various attributes of a relation.
It is used to define various normal forms.
These dependencies are restrictions imposed on the data in database.
If P is a relation with A and B attributes, a functional dependency between these two attributes is represented as {A → B}. It specifies that,
A
It is a determinant set.
B
It is a dependent attribute.
{A → B}
A functionally determines B. B is a functionally dependent on A.
Each value of A is associated precisely with one B value. A functional dependency is trivial if B is a subset of A.
'A' Functionality determines 'B' {A → B} (Left hand side attributes determine the values of Right hand side attributes).
For example: <Employee> Table
EmpId
EmpName
In the above <Employee> table, EmpName (employee name) is functionally dependent on EmpId (employee id) because the EmpId is unique for individual names.
The EmpId identifies the employee specifically, but EmpName cannot distinguish the EmpId because more than one employee could have the same name.
The functional dependency between attributes eliminates the repetition of information.
It is related to a candidate key, which uniquely identifies a tuple and determines the value of all other attributes in the relation.
Advantages of Functional Dependency
Functional Dependency avoids data redundancy where same data should not be repeated at multiple locations in same database.
It maintains the quality of data in database.
It allows clearly defined meanings and constraints of databases.
