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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:


    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 <EMPLOYEE_ZIP> table, with EMP_ZIP as a Primary key.

    EMPLOYEE table:

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

    EMPLOYEE_ZIP table:

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

    Next TopicDBMS BCNF

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