Second Normal Form (2NF) in SQL - Mini Project: Build & Apply

Organizing a Sales Database into Second Normal Form (2NF)

📖 Scenario: You are managing a small store's sales data. Currently, all sales information is stored in one table, but it has repeated data and some inefficiencies.Your task is to organize this data into Second Normal Form (2NF) to reduce redundancy and improve data integrity.

🎯 Goal: Create tables that follow Second Normal Form (2NF) by removing partial dependencies from the sales data.

📋 What You'll Learn

Create an initial table called Sales with columns OrderID, ProductID, ProductName, Quantity, and Price.

Add a primary key constraint on the combination of OrderID and ProductID.

Create a new table called Products with columns ProductID and ProductName to remove partial dependency.

Modify the Sales table to remove ProductName column after creating Products table.

💡 Why This Matters

🌍 Real World

Organizing sales data in a store database to avoid repeating product names in every order line.

💼 Career

Database designers and developers use normalization to create efficient, maintainable databases that reduce errors and improve performance.

1

Create the initial Sales table

Write a SQL statement to create a table called Sales with columns OrderID (integer), ProductID (integer), ProductName (text), Quantity (integer), and Price (decimal). Add a primary key on the combination of OrderID and ProductID.

SQL

-- Your SQL code to create the Sales table with the specified columns and composite primary key

Hint

Use CREATE TABLE with all columns and specify the primary key as (OrderID, ProductID).

2

Create the Products table

Write a SQL statement to create a new table called Products with columns ProductID (integer) and ProductName (text). Add a primary key on ProductID.

SQL

CREATE TABLE Sales (
  OrderID INT,
  ProductID INT,
  ProductName TEXT,
  Quantity INT,
  Price DECIMAL(10, 2),
  PRIMARY KEY (OrderID, ProductID)
);
-- Your SQL code to create the Products table with ProductID and ProductName

Hint

Use CREATE TABLE Products with ProductID as primary key and include ProductName.

3

Remove ProductName from Sales table

Write a SQL statement to remove the ProductName column from the Sales table to eliminate partial dependency.

SQL

CREATE TABLE Sales (
  OrderID INT,
  ProductID INT,
  ProductName TEXT,
  Quantity INT,
  Price DECIMAL(10, 2),
  PRIMARY KEY (OrderID, ProductID)
);

CREATE TABLE Products (
  ProductID INT PRIMARY KEY,
  ProductName TEXT
);
-- Your SQL code to remove ProductName column from Sales

Hint

Use ALTER TABLE Sales DROP COLUMN ProductName; to remove the column.

4

Add foreign key constraint to Sales

Write a SQL statement to add a foreign key constraint on ProductID in the Sales table that references ProductID in the Products table.

SQL

CREATE TABLE Sales (
  OrderID INT,
  ProductID INT,
  ProductName TEXT,
  Quantity INT,
  Price DECIMAL(10, 2),
  PRIMARY KEY (OrderID, ProductID)
);

CREATE TABLE Products (
  ProductID INT PRIMARY KEY,
  ProductName TEXT
);

ALTER TABLE Sales DROP COLUMN ProductName;
-- Your SQL code to add foreign key constraint on ProductID in Sales referencing Products

Hint

Use ALTER TABLE Sales ADD CONSTRAINT fk_ProductID FOREIGN KEY (ProductID) REFERENCES Products(ProductID);

Practice

1. Which of the following best describes the Second Normal Form (2NF) in database design?

easy

A. It removes partial dependencies on a part of a composite key.

B. It eliminates all transitive dependencies.

C. It requires that all columns be atomic.

D. It allows duplicate rows in a table.

Solution

1. Step 1: Understand 2NF definition

   2NF requires that no non-key attribute depends on only part of a composite primary key.

2. Step 2: Compare options with 2NF

   'It removes partial dependencies on a part of a composite key.' correctly states this. 'It eliminates all transitive dependencies.' describes 3NF. 'It requires that all columns be atomic.' describes 1NF, and 'It allows duplicate rows in a table.' is incorrect.

3. Final Answer:

   It removes partial dependencies on a part of a composite key. -> Option A

4. Quick Check:

   2NF = Remove partial dependencies [OK]

Hint: 2NF fixes partial dependencies on composite keys [OK]

Common Mistakes:

• Confusing 2NF with 3NF
• Thinking 2NF removes all duplicates
• Believing 2NF requires atomic columns

2. Which SQL table design violates Second Normal Form (2NF)?
CREATE TABLE Orders (OrderID INT, ProductID INT, ProductName VARCHAR(50), Quantity INT, PRIMARY KEY (OrderID, ProductID));

easy

A. OrderID and ProductID together form the primary key.

B. Quantity depends on both OrderID and ProductID.

C. ProductName depends only on ProductID, not the whole key.

D. The table has a composite primary key.

Solution

1. Step 1: Identify dependencies in the table

   ProductName depends only on ProductID, which is part of the composite key (OrderID, ProductID).

2. Step 2: Check 2NF violation

   Since ProductName depends on part of the key, this violates 2NF which forbids partial dependencies.

3. Final Answer:

   ProductName depends only on ProductID, not the whole key. -> Option C

4. Quick Check:

   Partial dependency = 2NF violation [OK]

Hint: Check if non-key columns depend on part of composite key [OK]

Common Mistakes:

• Assuming composite key alone ensures 2NF
• Ignoring partial dependencies
• Confusing primary key with foreign key

3. Given the table below:
StudentCourses(StudentID, CourseID, StudentName, CourseName, Grade)
Which of the following correctly identifies the partial dependencies violating 2NF?

medium

A. Grade depends only on StudentID.

B. StudentName depends only on StudentID, and CourseName depends only on CourseID.

C. StudentName depends on both StudentID and CourseID.

D. CourseName depends on Grade.

Solution

1. Step 1: Understand composite key and dependencies

   The composite key is (StudentID, CourseID). StudentName depends only on StudentID, CourseName only on CourseID.

2. Step 2: Check 2NF compliance

   2NF requires no partial dependencies. StudentName depending only on StudentID and CourseName only on CourseID are partial dependencies on parts of the composite key, violating 2NF.

3. Final Answer:

   StudentName depends only on StudentID, and CourseName depends only on CourseID. -> Option B

4. Quick Check:

   Partial dependencies exist if 2NF not met [OK]

Hint: In 2NF, no partial dependency on composite key parts [OK]

Common Mistakes:

• Thinking Grade depends only on StudentID
• Assuming CourseName depends on Grade
• Ignoring partial dependencies

4. You have this table:
CREATE TABLE Enrollment (StudentID INT, CourseID INT, StudentName VARCHAR(50), CourseName VARCHAR(50), PRIMARY KEY (StudentID, CourseID));
Which change fixes the 2NF violation?

medium

A. Keep the table as is; 2NF is not violated.

B. Add StudentName and CourseName as part of the primary key.

C. Remove the composite key and use a single primary key.

D. Split into three tables: Students(StudentID, StudentName), Courses(CourseID, CourseName), and Enrollment(StudentID, CourseID).

Solution

1. Step 1: Identify 2NF violation

   StudentName depends only on StudentID, CourseName only on CourseID, causing partial dependencies.

2. Step 2: Fix by splitting tables

   Splitting into Students(StudentID, StudentName), Courses(CourseID, CourseName), and Enrollment(StudentID, CourseID) removes partial dependencies, achieving 2NF.

3. Final Answer:

   Split into three tables: Students(StudentID, StudentName), Courses(CourseID, CourseName), and Enrollment(StudentID, CourseID). -> Option D

4. Quick Check:

   Split tables to remove partial dependencies [OK]

Hint: Split tables to fix partial dependencies [OK]

Common Mistakes:

• Adding non-key columns to primary key
• Removing composite key incorrectly
• Ignoring partial dependencies

5. A table has columns: OrderID, ProductID, ProductName, Quantity, SupplierName with composite primary key (OrderID, ProductID). SupplierName depends only on ProductID. How should you redesign the table to achieve 2NF?

hard

A. Create two tables: Orders(OrderID, ProductID, Quantity) and Products(ProductID, ProductName, SupplierName).

B. Add SupplierName to the primary key.

C. Remove ProductID from the primary key.

D. Keep all columns in one table; 2NF is already satisfied.

Solution

1. Step 1: Identify partial dependency

   SupplierName depends only on ProductID, part of the composite key, violating 2NF.

2. Step 2: Normalize by splitting tables

   Separate Products table with ProductID, ProductName, SupplierName removes partial dependency, Orders table keeps OrderID, ProductID, Quantity.

3. Final Answer:

   Create two tables: Orders(OrderID, ProductID, Quantity) and Products(ProductID, ProductName, SupplierName). -> Option A

4. Quick Check:

   Split tables to remove partial dependencies [OK]

Hint: Move partial dependency columns to separate table [OK]

Common Mistakes:

• Adding non-key columns to primary key
• Removing part of composite key incorrectly
• Assuming 2NF allows partial dependencies