What is database normalization? | SQL Interview Questions

What You'll Learn

What normalization is
Normal forms (1NF, 2NF, 3NF)
When to denormalize

What is Normalization?

Organizing data to reduce redundancy and improve data integrity.

First Normal Form (1NF)

Each column contains atomic (single) values
No repeating groups

query.sqlSQL

-- Bad (not 1NF)
| id | name | phones              |
|----|------|---------------------|
| 1  | John | 123-456, 789-012    |

-- Good (1NF)
| id | name | phone   |
|----|------|---------|
| 1  | John | 123-456 |
| 1  | John | 789-012 |

Second Normal Form (2NF)

Must be in 1NF
All non-key columns depend on the entire primary key

query.sqlSQL

-- Bad (not 2NF) - product_name depends only on product_id
| order_id | product_id | product_name | quantity |

-- Good (2NF) - separate tables
orders: | order_id | product_id | quantity |
products: | product_id | product_name |

Third Normal Form (3NF)

Must be in 2NF
No transitive dependencies (non-key depends on non-key)

query.sqlSQL

-- Bad (not 3NF) - city depends on zip_code, not on id
| id | name | zip_code | city    |

-- Good (3NF)
users: | id | name | zip_code |
locations: | zip_code | city |

When to Denormalize

Read-heavy applications
Reporting/analytics
Caching frequently joined data
Performance optimization

query.sqlSQL

-- Denormalized for performance
CREATE TABLE order_summary (
  order_id INT,
  user_name VARCHAR(100),  -- Copied from users
  product_names TEXT,       -- Aggregated
  total DECIMAL(10,2)
);