Third Normal Form (3NF)

What is Third Normal Form (3NF)?

3NF is a set of guidelines for organizing relational database tables to reduce data redundancy and improve data integrity. Achieving 3NF means your tables are well-structured and efficient. Here's what you need to know:

The Two Main Rules for 3NF

  1. The table must already be in Second Normal Form (2NF).

    • This means there are no partial dependencies; every non-key attribute is fully dependent on the whole primary key.

  2. There should be no transitive dependencies.

    • A transitive dependency happens when a non-key column depends on another non-key column, not directly on the primary key.

Transitive Dependency Example:
If column A determines column B, and column B determines column C, then A indirectly determines C. In 3NF, we want to remove these indirect relationships unless the dependencies are on a candidate key.

Example: Breaking Down a Table into 3NF

Suppose we have a table:
R (A, B, C)

With the following dependencies:

  • A → B
  • B → C

The Problem

  • A is our primary key.
  • B is a non-key attribute, but C depends on B, creating a transitive dependency (A → B → C).

The Original Table Layout

A B C
a 1 x
b 1 x
c 2 y
d 2 y
e 3 z
f 3 z

Here, column C does not depend directly on A, but on B.

How to Fix This: Splitting the Table

Step 1: Move the transitive dependency (B → C) into a separate table.

Table 1: (Remove the transitive dependency)

A B
a 1
b 1
c 2
d 2
e 3
f 3

Table 2: (Map B to C directly)

B C
1 x
2 y
3 z

Now, all non-key fields only depend on the key, directly—no more transitive dependencies. Congratulations, your tables are now in 3NF!

Why Does 3NF Matter?

Working in 3NF keeps your databases:

  • Efficient: Reduces repeated data (redundancy).
  • Consistent: Less risk of update anomalies.
  • Easy to maintain: Changes in one place, not many.

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