Second Normal Form (2NF) in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Second Normal Form (2NF) in database design?

2NF means a table is in First Normal Form (1NF) and all non-key columns depend on the whole primary key, not just part of it.

beginner

Why do we use Second Normal Form (2NF)?

To remove partial dependencies and avoid duplicate data, making the database easier to maintain and update.

intermediate

What is a partial dependency in the context of 2NF?

A partial dependency happens when a non-key column depends on only part of a composite primary key, not the whole key.

intermediate

Given a table with a composite key (StudentID, CourseID) and a column 'StudentName', why is this a problem for 2NF?

'StudentName' depends only on 'StudentID', part of the key, causing a partial dependency. This violates 2NF.

intermediate

How do you fix a table that violates 2NF?

Split the table into two: one with the full key and dependent columns, and another with the partial key and its dependent columns.

What must a table already satisfy before it can be in Second Normal Form (2NF)?

A It must be in First Normal Form (1NF)

B It must have no null values

C It must have only one column

D It must have no foreign keys

2NF requires the table to be in 1NF first, meaning no repeating groups and atomic columns.

What does 2NF primarily aim to eliminate?

A Partial dependencies

B Transitive dependencies

C Duplicate rows

D Null values

2NF removes partial dependencies where columns depend on part of a composite key.

If a table has a single-column primary key, is it automatically in 2NF?

A No, it must be in 3NF first

B No, it must have multiple keys

C Yes, but only if it has no nulls

D Yes, because no partial dependency is possible

With a single-column key, partial dependencies cannot exist, so 2NF is automatically satisfied.

Which of these is an example of a partial dependency?

A A column depends on the entire primary key

B A column depends on only part of a composite key

C A column depends on a non-key column

D A column has null values

Partial dependency means a column depends on only part of a composite key.

How can you fix a partial dependency in a table?

A Remove the primary key

B Add more columns to the table

C Split the table into two tables to separate dependencies

D Add null values to dependent columns

Splitting the table removes partial dependencies by isolating columns that depend on part of the key.

Explain Second Normal Form (2NF) and why it is important in database design.

Think about how columns relate to the whole primary key.

Describe how you would identify and fix a partial dependency in a table with a composite primary key.

Focus on which columns depend on which parts of the key.

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