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    Third Normal Form (3NF)

    Published Jun 21, 2025 [  Database  ]

    Third Normal Form (3NF) is a database normalization standard used to reduce redundancy and ensure data integrity. A table is in 3NF if:

    1. It is in Second Normal Form (2NF), and
    2. All non-prime attributes are non-transitively dependent on the primary key.

    Terms

    • Non-prime attribute: An attribute that is not part of any candidate key.
    • Transitive dependency: A situation where attribute A depends on B, and B depends on C, so A depends on C indirectly.

    Example

    Not in 3NF

    StudentID Name Department DepartmentHead
    1 Alice CS Dr. Smith
    2 Bob EE Dr. Jones

    Here:

    • StudentID → Department
    • Department → DepartmentHead

    So, StudentID → DepartmentHead is a transitive dependency via Department.

    In 3NF

    Split into two tables:

    Students

    StudentID Name Department
    1 Alice CS
    2 Bob EE

    Departments

    Department DepartmentHead
    CS Dr. Smith
    EE Dr. Jones

    Now, all non-prime attributes depend directly on the primary key, not transitively.