Second Normal Form (2NF) in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query to select all columns from the table named 'Orders'.

SQL

SELECT [1] FROM Orders;

Drag options to blanks, or click blank then click option'

A*

BALL

CEVERYTHING

DCOLUMNS

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ALL' instead of '*' to select all columns.

Using 'COLUMNS' or 'EVERYTHING' which are not valid SQL keywords.

The asterisk (*) is used in SQL to select all columns from a table.

2fill in blank

medium

Complete the SQL query to find orders where the quantity is greater than 10.

SQL

SELECT * FROM Orders WHERE quantity [1] 10;

Drag options to blanks, or click blank then click option'

A=

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which checks for equality, not greater than.

Using '<' or '<=' which check for less than conditions.

The '>' operator is used to find values greater than a specified number.

3fill in blank

hard

Fix the error in the SQL query to select distinct customer IDs from Orders.

SQL

SELECT [1] customer_id FROM Orders;

Drag options to blanks, or click blank then click option'

ADISTINCT

BUNIQUE

CSINGLE

DONLY

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'UNIQUE' which is not valid in SELECT clauses for distinct values.

Using 'SINGLE' or 'ONLY' which are not SQL keywords.

The keyword 'DISTINCT' is used in SQL to return unique values.

4fill in blank

hard

Fill both blanks to create a SQL query that groups orders by customer_id and counts the number of orders.

SQL

SELECT customer_id, COUNT([1]) AS order_count FROM Orders GROUP BY [2];

Drag options to blanks, or click blank then click option'

A*

Border_id

Ccustomer_id

Dquantity

Attempts:

3 left

💡 Hint

Common Mistakes

Counting a specific column that might have NULLs instead of all rows.

Grouping by a column other than customer_id.

COUNT(*) counts all rows. GROUP BY customer_id groups the results by each customer.

5fill in blank

hard

Fill all three blanks to write a SQL query that selects product_id, sums quantity sold, and groups by product_id having total quantity greater than 100.

SQL

SELECT [1], SUM([2]) AS total_quantity FROM Sales GROUP BY [3] HAVING total_quantity > 100;

Drag options to blanks, or click blank then click option'

Aproduct_id

Bquantity

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by order_id instead of product_id.

Summing a wrong column or not grouping properly.

Select product_id, sum the quantity sold, group by product_id to aggregate, and filter groups with total quantity over 100.

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