First Normal Form (1NF): atomic values and repeating groups

First Normal Form Rules

A table is in First Normal Form (1NF) when every cell contains exactly one value (atomic), every row is unique, and there are no repeating column groups.

Common Violations

Multi-value cells: Storing "Python, Java, SQL" in a single skills column. You cannot query for all developers who know Java without a LIKE hack.

Repeating column groups: Creating phone1, phone2, phone3 columns. You cannot support a fourth phone number without altering the schema.

How to Fix

-- Violates 1NF: multi-value cell
CREATE TABLE employees_bad (
  emp_id INT PRIMARY KEY,
  name   VARCHAR(100),
  skills VARCHAR(255)  -- 'Python,Java,SQL'
);

-- Satisfies 1NF: separate table
CREATE TABLE employees (
  emp_id INT PRIMARY KEY,
  name   VARCHAR(100) NOT NULL
);

CREATE TABLE employee_skills (
  emp_id     INT NOT NULL,
  skill_name VARCHAR(50) NOT NULL,
  PRIMARY KEY (emp_id, skill_name),
  FOREIGN KEY (emp_id) REFERENCES employees(emp_id)
);

The fix always follows the same pattern: extract the multi-valued attribute into its own table with a foreign key back to the parent. The composite PK on the new table enforces uniqueness automatically.