First Normal Form (1NF) in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main rule of First Normal Form (1NF) in databases?

1NF requires that each column in a table contains only atomic (indivisible) values, and each record is unique with no repeating groups or arrays.

beginner

Why should a table not have repeating groups or arrays in 1NF?

Repeating groups or arrays make data hard to query and update. 1NF avoids this by ensuring each field holds only one value, making data simpler and consistent.

beginner

Which of these violates 1NF?

StudentID | Courses
1         | Math, Science
2         | English

This violates 1NF because the 'Courses' column contains multiple values (Math, Science) in one field. Each field should have only one value.

intermediate

How can you fix a table that violates 1NF due to multiple values in one column?

You can create a separate row for each value or use a related table to store multiple values, so each field holds only one atomic value.

beginner

What does it mean for a value to be atomic in the context of 1NF?

Atomic means the value cannot be divided further. For example, a single phone number is atomic, but a list of phone numbers in one field is not.

Which of the following is a violation of First Normal Form (1NF)?

A A column contains multiple phone numbers separated by commas

B Each row has a unique ID

C Each column contains only one value

D No duplicate rows exist

1NF requires each column to have atomic values. Multiple phone numbers in one field violate this.

What does 1NF ensure about the structure of a database table?

A Tables have indexes

B Foreign keys are used properly

C Data is encrypted

D No repeating groups and atomic column values

1NF focuses on eliminating repeating groups and ensuring atomic values in columns.

How can you correct a table that stores multiple values in one column to meet 1NF?

A Add more columns with the same values

B Split the values into separate rows or related tables

C Use a comma-separated string in the same column

D Ignore it, 1NF does not care

Splitting values into separate rows or tables ensures atomicity and meets 1NF.

Which of these is an atomic value suitable for 1NF?

A A single email address

B A list of hobbies separated by commas

C Multiple phone numbers in one field

D An array of tags

An atomic value is indivisible, like a single email address.

What problem does 1NF help to avoid?

A Lack of user permissions

B Slow network connections

C Data redundancy and complex queries due to multi-valued fields

D Incorrect data types

1NF avoids multi-valued fields that cause redundancy and make queries complex.

Explain in your own words what First Normal Form (1NF) means and why it is important.

Think about how data should be stored in each column.

Describe a real-life example where a table violates 1NF and how you would fix it.

Imagine a list of phone numbers stored in one cell.

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