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Database Normalization: 3NF

Third Normal Form (3NF) is a database normalization step that ensures your tables are structured to reduce redundancy and improve data integrity, building on the first (1NF) and second normal forms (2NF).

Quick Recap Before 3NF

  1. 1NF (First Normal Form) – No repeating groups, atomic column values, each row is unique.
  2. 2NF (Second Normal Form) – In 1NF, and every non-key attribute is fully dependent on the whole primary key (no partial dependency).

Definition of 3NF

A table is in Third Normal Form if:

  1. It is already in 2NF, and

  2. It has no transitive dependencies, meaning:

    • Non-key attributes should depend only on the primary key, not on another non-key attribute.

In plain terms: Every column should tell you something about the key, the whole key, and nothing but the key.

Example

Before 3NF (in 2NF but not in 3NF)

Orders
-------------------------------------------------
OrderID | CustomerID | CustomerName | CustomerCity
-------------------------------------------------
1       | C001       | Alice        | New York
2       | C002       | Bob          | Los Angeles
3       | C001       | Alice        | New York

Problem:

  • CustomerName and CustomerCity depend on CustomerID, not directly on OrderID.
  • This is a transitive dependency: OrderID → CustomerID → CustomerName/City.

After 3NF

We split into two tables:

Orders

OrderID | CustomerID
--------------------
1       | C001
2       | C002
3       | C001

Customers

CustomerID | CustomerName | CustomerCity
----------------------------------------
C001       | Alice        | New York
C002       | Bob          | Los Angeles

Benefits:

  • Avoids storing customer details repeatedly.
  • Updating a customer’s city requires changing only one row in Customers.

Why 3NF Matters

✅ Reduces data duplication ✅ Makes updates faster and less error-prone ✅ Improves data integrity ⚠️ Sometimes too much normalization can hurt performance for read-heavy queries — you might need joins.