First Normal Form (1NF) in SQL

Introduction

First Normal Form (1NF) helps organize data in a table so that each piece of information is stored clearly and separately. This makes data easier to understand and use.

When you want to make sure each column in a table holds only one value.

When you notice a table has columns with multiple values in one cell.

When you want to avoid confusion and errors in your data by keeping it simple.

When preparing data for easy searching and updating.

When designing a new database to keep data clean from the start.

Syntax

SQL

CREATE TABLE table_name (
  column1 datatype PRIMARY KEY,
  column2 datatype NOT NULL,
  column3 datatype,
  ...
);

-- Ensure each column holds atomic (single) values only.
-- No repeating groups or arrays in columns.

1NF means each column must have atomic values, which means no lists or multiple values in one cell.

Each row should be unique, often ensured by a primary key.

Examples

A simple table where each column holds one value per row, meeting 1NF.

SQL

CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  Name VARCHAR(100),
  Age INT
);

Each order has one product and quantity per row, no multiple products in one cell.

SQL

CREATE TABLE Orders (
  OrderID INT PRIMARY KEY,
  ProductName VARCHAR(100),
  Quantity INT
);

PhoneNumber column holds one phone number per employee, not multiple numbers in one cell.

SQL

CREATE TABLE Employees (
  EmployeeID INT PRIMARY KEY,
  Name VARCHAR(100),
  PhoneNumber VARCHAR(15)
);

Sample Program

This creates a Books table that follows 1NF by storing one book title and one author per row.

SQL

CREATE TABLE Books (
  BookID INT PRIMARY KEY,
  Title VARCHAR(100),
  Author VARCHAR(100)
);

INSERT INTO Books (BookID, Title, Author) VALUES
(1, 'The Cat in the Hat', 'Dr. Seuss'),
(2, 'Green Eggs and Ham', 'Dr. Seuss');

SELECT * FROM Books;

Important Notes

If a column has multiple values, split them into separate rows or columns.

Always choose a primary key to uniquely identify each row.

1NF is the first step to organizing your data well before moving to more advanced forms.

Summary

1NF means each table column holds only one value per row.

No lists or multiple values allowed in a single cell.

Helps keep data simple, clear, and easy to work with.

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