Keys and joins

New idea

Sara's phone, written 40 times

A shop keeps one big orders table, and every order row repeats the customer's name and phone. Sara has ordered 40 times, so her phone number is stored 40 times.

Sara gets a new number. Now 40 rows must change.

The fix: store each fact once. Customers go in a customers table, orders in an orders table, and each order only says which customer it belongs to.