**Third normal form (3NF)**

- A table is said to be in the
**Third Normal Form**, if all the non key fields of the table are independent of all other non key fields of the table.
- A relation is in the third normal form if it is in second normal form and no non-prime attribute is functionally dependent on other non-prime attribute.
- The 2NF tables we established in the previous section represent a significant improvement over 1NF tables.
- However, they still suffer from the same types of anomalies as the 1NF tables although for different reasons associated with transitive dependencies.
- If a transitive (functional) dependency exists in a table, it means that two separate facts are represented in that table, one fact for each functional dependency involving a different left side.
- A table is in third normal form (3NF) if and only if for every nontrivial functional dependency X->A, where X and A are either simple or composite attributes, one of two conditions must hold.
- Either attribute X is a superkey, or attribute A is a member of a candidate key.
- If attribute A is a member of a candidate key, A is called a prime attribute.
- A relation will be in 3NF if it is in 2NF and not contain any transitive partial dependency.
- 3NF is used to reduce the data duplication. It is also used to achieve the data integrity.
- If there is no transitive dependency for non-prime attributes, then the relation must be in third normal form.