﻿ Third Normal Form (3NF), | Online Tutorials Library | Tutorialsinfo.com

# Third Normal Form (3NF)

• 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.

A relation is in third normal form if it holds atleast one of the following conditions for every non-trivial function dependency X → Y.

1. X is a super key.
2. Y is a prime attribute, i.e., each element of Y is part of some candidate key.
3. Example:

EMPLOYEE_DETAIL table:

EMP_ID EMP_NAME EMP_ZIP EMP_STATE EMP_CITY
222 Harry 201010 UP Noida
333 Stephan 02228 US Boston
444 Lan 60007 US Chicago
555 Katharine 06389 UK Norwich
666 John 462007 MP Bhopal

Super key in the table above:

Candidate key: {EMP_ID}

Non-prime attributes: In the given table, all attributes except EMP_ID are non-prime.

Here, EMP_STATE & EMP_CITY dependent on EMP_ZIP and EMP_ZIP dependent on EMP_ID. The non-prime attributes (EMP_STATE, EMP_CITY) transitively dependent on super key(EMP_ID). It violates the rule of third normal form.

That's why we need to move the EMP_CITY and EMP_STATE to the new table, with EMP_ZIP as a Primary key.

EMPLOYEE table:

EMP_ID EMP_NAME EMP_ZIP
222 Harry 201010
333 Stephan 02228
444 Lan 60007
555 Katharine 06389
666 John 462007

EMPLOYEE_ZIP table:

EMP_ZIP EMP_STATE EMP_CITY
201010 UP Noida
02228 US Boston
60007 US Chicago
06389 UK Norwich
462007 MP Bhopal

Next TopicDBMS BCNF