What is functional dependency?
- 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,
- 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).
- 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.
| 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. |
For example: <Employee> Table
| EmpId | EmpName |
|---|---|
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.
- It helps in identifying bad designs.
- It expresses the facts about the database design.
