Normalization in Plain English: 1NF, 2NF, 3NF

The goal

Normalization stores each fact once, so an update happens in one place and the data cannot contradict itself.

Start with a messy table

1NF — one value per cell

Split repeating values into rows:

2NF — every column depends on the whole key

With key (order_id, product), customer depends only on order_id. Move it out:

orders(order_id, customer)
order_lines(order_id, product)

3NF — no column depends on another non-key column

customer_city depends on the customer, not on the order:

customers(customer_id, name, city)
orders(order_id, customer_id)
order_lines(order_id, line_no, product_id, qty)

Now a customer moving city changes one row.

Why warehouses denormalize again

Analytical queries would need many joins on a 3NF schema. A star schema flattens descriptive attributes into wide dimensions (city, region and country all in dim_customer). That trades some redundancy for simple, fast queries.