Third Normal Form (3NF) in SQL - Step-by-Step Execution

Concept Flow - Third Normal Form (3NF)

Start with a Table

↓

Check 1NF: Atomic values?

Yes↓

Check 2NF: No partial dependencies?

Yes↓

Check 3NF: No transitive dependencies?

Yes↓

Result: Tables in 3NF

Start with a table, ensure it meets 1NF and 2NF, then remove transitive dependencies to achieve 3NF.

Execution Sample

SQL

CREATE TABLE Orders (
  OrderID INT PRIMARY KEY,
  CustomerID INT,
  CustomerName VARCHAR(100),
  OrderDate DATE
);

This table has a transitive dependency: CustomerName depends on CustomerID, not directly on OrderID.

Execution Table

Step	Check	Condition	Result	Action
1	Check 1NF	Are all values atomic?	Yes	Proceed to 2NF check
2	Check 2NF	Are there partial dependencies?	No	Proceed to 3NF check
3	Check 3NF	Are there transitive dependencies?	Yes	Decompose table
4	Decompose	Split Orders into Orders and Customers	Orders(OrderID, CustomerID, OrderDate), Customers(CustomerID, CustomerName)	Tables now in 3NF
5	Final Check	Are all tables in 3NF?	Yes	Normalization complete

💡 All tables satisfy 3NF after decomposition, no transitive dependencies remain.

Variable Tracker

Table	Initial Columns	After Decomposition	Final Columns
Orders	OrderID, CustomerID, CustomerName, OrderDate	Orders: OrderID, CustomerID, OrderDate Customers: CustomerID, CustomerName	Orders: OrderID, CustomerID, OrderDate Customers: CustomerID, CustomerName

Key Moments - 2 Insights

Why do we remove CustomerName from the Orders table?

Because CustomerName depends on CustomerID, not directly on OrderID, causing a transitive dependency (see execution_table step 3). Removing it breaks this dependency.

What does it mean for a table to have no transitive dependencies?

It means non-key columns depend only on the primary key, not on other non-key columns (refer to execution_table step 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, at which step do we find transitive dependencies?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Check 3NF' row in execution_table where transitive dependencies are identified.

According to variable_tracker, what columns does the Customers table have after decomposition?

A CustomerID, CustomerName

B OrderID, CustomerID, CustomerName

C OrderID, OrderDate

D CustomerName, OrderDate

💡 Hint

Look at the 'After Decomposition' and 'Final Columns' rows for Customers in variable_tracker.

If CustomerName stayed in Orders, what problem would remain?

A Partial dependency

B Atomicity violation

C Transitive dependency

D Primary key missing

💡 Hint

Refer to execution_table step 3 where transitive dependencies are detected.

Concept Snapshot

Third Normal Form (3NF):
- Table must be in 2NF first.
- Remove transitive dependencies: no non-key column depends on another non-key column.
- Decompose tables to separate related data.
- Result: each non-key attribute depends only on the primary key.
- Ensures data integrity and reduces redundancy.

Full Transcript

Third Normal Form (3NF) is a database design principle to organize tables so that every non-key column depends only on the primary key. We start by ensuring the table is in First Normal Form (1NF) with atomic values, then Second Normal Form (2NF) with no partial dependencies. Next, we check for transitive dependencies, where a non-key column depends on another non-key column. If found, we split the table into smaller tables to remove these dependencies. For example, in an Orders table containing CustomerName, since CustomerName depends on CustomerID and not directly on OrderID, we separate Customers into its own table. This process reduces redundancy and improves data integrity. The execution table shows each step, and the variable tracker shows how columns move between tables during decomposition.

Practice

1. Which of the following best describes Third Normal Form (3NF) in database design?

easy

A. Every non-key column depends only on the primary key.

B. Tables must have no more than three columns.

C. All columns must be unique across the table.

D. Data must be stored in a single table without duplicates.

Solution

1. Step 1: Understand 3NF definition

   3NF requires that non-key columns depend only on the primary key, not on other non-key columns.

2. Step 2: Evaluate options

   Every non-key column depends only on the primary key. correctly states this. The other options are incorrect descriptions of 3NF.

3. Final Answer:

   Every non-key column depends only on the primary key. -> Option A

4. Quick Check:

   3NF = Non-key columns depend only on primary key [OK]

Hint: 3NF means no column depends on non-key columns [OK]

Common Mistakes:

• Confusing 3NF with limiting number of columns
• Thinking all columns must be unique
• Believing 3NF means one big table

2. Which SQL statement correctly creates a table that follows Third Normal Form (3NF)?

easy

A. CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT, CustomerName VARCHAR(50));

B. CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerName VARCHAR(50), CustomerAddress VARCHAR(100));

C. CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT); CREATE TABLE Customers (CustomerID INT PRIMARY KEY, CustomerName VARCHAR(50));

D. CREATE TABLE Orders (OrderID INT, CustomerName VARCHAR(50));

Solution

1. Step 1: Identify 3NF compliance in table design

   3NF requires separating data so non-key columns do not depend on other non-key columns. CustomerName depends on CustomerID, so it should be in a separate table.

2. Step 2: Analyze options

   CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT); CREATE TABLE Customers (CustomerID INT PRIMARY KEY, CustomerName VARCHAR(50)); splits Orders and Customers, following 3NF. CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT, CustomerName VARCHAR(50)); stores CustomerName in Orders, violating 3NF. The other options miss primary keys or mix data improperly.

3. Final Answer:

   CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT); CREATE TABLE Customers (CustomerID INT PRIMARY KEY, CustomerName VARCHAR(50)); -> Option C

4. Quick Check:

   Separate related data into tables = 3NF [OK]

Hint: Split tables to avoid non-key dependencies [OK]

Common Mistakes:

• Putting dependent data in the same table
• Missing primary keys
• Ignoring data duplication issues

3. Given the table below, which is NOT in Third Normal Form (3NF)?

EmployeeID | EmployeeName | Department | DepartmentHead

What is the main reason this table violates 3NF?

medium

A. DepartmentHead depends on Department, not EmployeeID.

B. EmployeeName depends on EmployeeID.

C. EmployeeID is not unique.

D. Department is a primary key.

Solution

1. Step 1: Identify dependencies in the table

   DepartmentHead depends on Department, which is a non-key column, not on the primary key EmployeeID.

2. Step 2: Check 3NF rules

   3NF requires no non-key column depends on another non-key column. Here, DepartmentHead depends on Department, violating 3NF.

3. Final Answer:

   DepartmentHead depends on Department, not EmployeeID. -> Option A

4. Quick Check:

   Non-key depends on non-key = violates 3NF [OK]

Hint: Check if non-key columns depend on other non-key columns [OK]

Common Mistakes:

• Thinking EmployeeName dependency breaks 3NF
• Assuming EmployeeID is not unique
• Confusing primary key with foreign key

4. You have a table:

StudentID | StudentName | Advisor | AdvisorOffice

This table violates 3NF. What is the best way to fix it?

medium

A. Remove AdvisorOffice column.

B. Create a separate Advisor table with Advisor and AdvisorOffice.

C. Make StudentName the primary key.

D. Add AdvisorOffice to StudentID as a foreign key.

Solution

1. Step 1: Identify the 3NF violation

   AdvisorOffice depends on Advisor, a non-key column, causing a transitive dependency violating 3NF.

2. Step 2: Fix by separating dependent data

   Creating a separate Advisor table with Advisor and AdvisorOffice removes the transitive dependency.

3. Final Answer:

   Create a separate Advisor table with Advisor and AdvisorOffice. -> Option B

4. Quick Check:

   Separate dependent data into own table to fix 3NF [OK]

Hint: Fix 3NF by splitting tables for transitive dependencies [OK]

Common Mistakes:

• Just removing columns without restructuring
• Changing primary keys incorrectly
• Adding foreign keys without normalization

5. A company has a table:

InvoiceID | ProductID | ProductName | SupplierID | SupplierName

To convert this table into Third Normal Form (3NF), which of the following is the best approach?

hard

A. Remove SupplierName column from the table.

B. Add ProductName and SupplierName as primary keys.

C. Keep all columns in one table but add unique constraints.

D. Create separate tables for Products and Suppliers, linking them by IDs.

Solution

1. Step 1: Identify transitive dependencies

   ProductName depends on ProductID, and SupplierName depends on SupplierID, both non-key columns, causing 3NF violation.

2. Step 2: Normalize by splitting tables

   Creating separate Product and Supplier tables linked by IDs removes these dependencies and achieves 3NF.

3. Final Answer:

   Create separate tables for Products and Suppliers, linking them by IDs. -> Option D

4. Quick Check:

   Split related entities into tables to achieve 3NF [OK]

Hint: Split entities into tables linked by keys for 3NF [OK]

Common Mistakes:

• Trying to keep all data in one table
• Using names as primary keys
• Removing columns without restructuring