First Normal Form (1NF) in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the table named 'students'.

SQL

SELECT [1] FROM students;

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

AEVERY

BALL

C*

DALL_COLUMNS

Attempts:

3 left

💡 Hint

Common Mistakes

Using keywords like ALL or EVERY which are not valid for selecting all columns.

The asterisk (*) selects all columns from the table.

2fill in blank

medium

Complete the code to select the 'name' and 'phone_numbers' columns from the 'contacts' table.

SQL

SELECT name, [1] FROM contacts;

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

Aphone_numbers

Bphone_number

Cphones

Dphone

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing singular form 'phone_number' when the column is named 'phone_numbers'.

The column name is 'phone_numbers' which may contain multiple phone numbers, but in 1NF, each value should be atomic.

3fill in blank

hard

Fix the error in the query that tries to select rows where the 'phone_numbers' column contains multiple values, which violates 1NF.

SQL

SELECT * FROM contacts WHERE phone_numbers [1] '%,%';

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

AIN

BLIKE

C=

DBETWEEN

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which checks for exact match, not pattern matching.

The LIKE operator checks if the 'phone_numbers' string contains a comma, indicating multiple values in one field, which violates 1NF.

4fill in blank

hard

Fill both blanks to create a query that selects 'id' and 'phone' from 'contacts' where 'phone' is atomic (does not contain commas).

SQL

SELECT id, [1] FROM contacts WHERE [2] NOT LIKE '%,%';

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

Aphone

Bphone_numbers

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'phone_numbers' which may contain multiple values.

We select the atomic 'phone' column and filter rows where 'phone' does not contain commas, ensuring 1NF compliance.

5fill in blank

hard

Fill all three blanks to create a query that creates a new table 'contacts_1nf' with atomic phone numbers by splitting 'phone_numbers' into single values.

SQL

CREATE TABLE contacts_1nf AS SELECT id, [1] AS phone FROM contacts, [2](phone_numbers) AS [3] WHERE phone IS NOT NULL;

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

Aphone

Bunnest

Dsplit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'split' which is not a standard SQL function for this purpose.

The 'unnest' function is used to split array-like 'phone_numbers' into single 'phone' values, creating a 1NF-compliant table.

Practice

1. What does First Normal Form (1NF) require in a database table?

easy

A. Each table must have a primary key.

B. Tables must have at least two columns.

C. Each column must contain atomic (single) values only.

D. Columns can contain lists or multiple values.

Solution

1. Step 1: Understand 1NF definition

   1NF requires that each column holds only one atomic value per row, no lists or sets.

2. Step 2: Evaluate options

   Each column must contain atomic (single) values only. matches the 1NF rule exactly. Options A, B, and D describe other concepts or violate 1NF.

3. Final Answer:

   Each column must contain atomic (single) values only. -> Option C

4. Quick Check:

   1NF = atomic columns only [OK]

Hint: 1NF means no multiple values in one cell [OK]

Common Mistakes:

• Thinking 1NF requires a primary key
• Allowing lists or sets in columns
• Confusing 1NF with other normal forms

2. Which of the following SQL table definitions violates First Normal Form (1NF)?

easy

A. CREATE TABLE Students (ID INT, Name VARCHAR(50), PhoneNumbers VARCHAR(100));

B. CREATE TABLE Students (ID INT PRIMARY KEY, Name VARCHAR(50), PhoneNumber VARCHAR(15));

C. CREATE TABLE Students (ID INT, Name VARCHAR(50), Email VARCHAR(50));

D. CREATE TABLE Students (ID INT, Name VARCHAR(50), Address VARCHAR(100));

Solution

1. Step 1: Identify 1NF violation in table definitions

   CREATE TABLE Students (ID INT, Name VARCHAR(50), PhoneNumbers VARCHAR(100)); uses PhoneNumbers as VARCHAR(100), likely storing multiple numbers in one field, violating 1NF.

2. Step 2: Check other options

   Options A, B, and C have single atomic values per column, so they follow 1NF.

3. Final Answer:

   CREATE TABLE Students (ID INT, Name VARCHAR(50), PhoneNumbers VARCHAR(100)); -> Option A

4. Quick Check:

   Multiple values in one column break 1NF [OK]

Hint: Multiple values in one column break 1NF [OK]

Common Mistakes:

• Assuming VARCHAR can hold multiple values without issue
• Confusing primary key with 1NF rules
• Ignoring column data types when checking 1NF

3. Given the table below, which row violates First Normal Form (1NF)?

Orders(OrderID, Customer, Items)
Where Items column contains comma-separated product IDs.

Rows:
1. (101, 'Alice', 'P1')
2. (102, 'Bob', 'P2,P3')
3. (103, 'Carol', 'P4')
4. (104, 'Dave', 'P5')

medium

A. Row 1

B. Row 2

C. Row 3

D. Row 4

Solution

1. Step 1: Understand 1NF violation in data rows

   Row 2 has 'P2,P3' in Items, which is multiple values in one cell, violating 1NF.

2. Step 2: Check other rows

   Rows 1, 3, and 4 have single product IDs, so they follow 1NF.

3. Final Answer:

   Row 2 -> Option B

4. Quick Check:

   Multiple values in one cell break 1NF [OK]

Hint: Comma-separated values in one cell break 1NF [OK]

Common Mistakes:

• Ignoring multiple values separated by commas
• Assuming string with commas is atomic
• Not checking all rows carefully

4. You have this table definition violating 1NF:

CREATE TABLE Employees (ID INT, Name VARCHAR(50), Skills VARCHAR(100));

Skills column stores multiple skills separated by commas.
Which SQL change fixes the 1NF violation?

medium

A. Store skills as a single string with semicolons instead of commas.

B. Change Skills to TEXT and store JSON arrays.

C. Keep Skills as VARCHAR but limit length to 50.

D. Create a separate Skills table with EmployeeID and Skill columns.

Solution

1. Step 1: Identify 1NF violation and fix approach

   Storing multiple skills in one column violates 1NF. The fix is to separate repeating data into another table.

2. Step 2: Evaluate options for 1NF compliance

   Create a separate Skills table with EmployeeID and Skill columns. creates a separate Skills table with one skill per row, fixing 1NF. Options B, C, and D still store multiple values in one column.

3. Final Answer:

   Create a separate Skills table with EmployeeID and Skill columns. -> Option D

4. Quick Check:

   Separate repeating data into new table for 1NF [OK]

Hint: Use separate table for repeating groups to fix 1NF [OK]

Common Mistakes:

• Trying to store multiple values in one column anyway
• Using JSON or delimiters inside one column
• Not normalizing repeating data into separate tables

5. You have a table storing orders with multiple products in one column:

Orders(OrderID INT, Customer VARCHAR(50), Products VARCHAR(255))
Products column stores product IDs separated by commas.

How do you redesign the schema to fully comply with First Normal Form (1NF)?

hard

A. Create a new table OrderProducts(OrderID, ProductID) with one product per row.

B. Split Products into multiple columns like Product1, Product2, Product3.

C. Store Products as JSON array in the same column.

D. Keep Products column but restrict to one product ID only.

Solution

1. Step 1: Identify 1NF violation in current design

   Storing multiple product IDs in one column violates 1NF because values are not atomic.

2. Step 2: Choose redesign that enforces atomicity

   Create a new table OrderProducts(OrderID, ProductID) with one product per row. creates a new table with one product per row, fully normalizing the data and satisfying 1NF. Split Products into multiple columns like Product1, Product2, Product3. uses multiple columns but is limited and not scalable. Options C and D still violate 1NF.

3. Final Answer:

   Create a new table OrderProducts(OrderID, ProductID) with one product per row. -> Option A

4. Quick Check:

   Separate repeating data into rows for 1NF compliance [OK]

Hint: Use separate table with one value per row for 1NF [OK]

Common Mistakes:

• Adding multiple columns instead of rows
• Using JSON or arrays inside one column
• Restricting to one product but losing data