Third Normal Form (3NF) in SQL - Mini Project: Build & Apply

Organizing a Library Database in Third Normal Form (3NF)

📖 Scenario: You are helping a small library organize its book records. The library currently stores all information in one big table, which makes it hard to update and causes repeated data. You will break this big table into smaller, well-structured tables following the Third Normal Form (3NF) rules.

🎯 Goal: Create three separate tables: Books, Authors, and Publishers. Each table should have the right columns so that data is not repeated and each fact is stored only once. This will make the library database easier to maintain and update.

📋 What You'll Learn

Create a table called Books with columns BookID, Title, AuthorID, and PublisherID.

Create a table called Authors with columns AuthorID and AuthorName.

Create a table called Publishers with columns PublisherID and PublisherName.

Use AuthorID and PublisherID as foreign keys in the Books table to link to the other tables.

Ensure each table has a primary key to uniquely identify each record.

💡 Why This Matters

🌍 Real World

Libraries, bookstores, and many businesses use normalized databases to keep their data clean and easy to update.

💼 Career

Database designers and developers must understand normalization to build efficient and reliable databases.

1

Create the initial Books table

Write a SQL statement to create a table called Books with columns BookID as an integer primary key, Title as text, AuthorID as integer, and PublisherID as integer.

SQL

-- Create the Books table with BookID, Title, AuthorID, and PublisherID
-- Your code here

Hint

Use CREATE TABLE Books and define each column with its type. Mark BookID as the primary key.

2

Create the Authors table

Write a SQL statement to create a table called Authors with columns AuthorID as an integer primary key and AuthorName as text.

SQL

CREATE TABLE Books (
  BookID INT PRIMARY KEY,
  Title TEXT,
  AuthorID INT,
  PublisherID INT
);
-- Create the Authors table with AuthorID and AuthorName
-- Your code here

Hint

Define Authors with AuthorID as primary key and AuthorName as text.

3

Create the Publishers table

Write a SQL statement to create a table called Publishers with columns PublisherID as an integer primary key and PublisherName as text.

SQL

CREATE TABLE Books (
  BookID INT PRIMARY KEY,
  Title TEXT,
  AuthorID INT,
  PublisherID INT
);

CREATE TABLE Authors (
  AuthorID INT PRIMARY KEY,
  AuthorName TEXT
);
-- Create the Publishers table with PublisherID and PublisherName
-- Your code here

Hint

Define Publishers with PublisherID as primary key and PublisherName as text.

4

Add foreign key constraints to Books table

Modify the Books table to add foreign key constraints on AuthorID referencing Authors(AuthorID) and on PublisherID referencing Publishers(PublisherID).

SQL

CREATE TABLE Books (
  BookID INT PRIMARY KEY,
  Title TEXT,
  AuthorID INT,
  PublisherID INT,
  -- Add foreign key constraints here
);

CREATE TABLE Authors (
  AuthorID INT PRIMARY KEY,
  AuthorName TEXT
);

CREATE TABLE Publishers (
  PublisherID INT PRIMARY KEY,
  PublisherName TEXT
);

Hint

Use FOREIGN KEY clauses inside the Books table to link AuthorID and PublisherID to their tables.

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