Why One Table Is Rarely Enough

Imagine tracking a shop's sales in a single table, with columns for CustomerName, CustomerPhone, ProductTitle, ProductPrice, and OrderDate. The moment a customer places a second order, their name and phone number get typed in all over again, on a brand-new row. If that customer's phone number ever changes, you now have to find and fix it in every single one of their past orders — and it's easy to miss one, leaving inconsistent, contradictory data behind. This problem is called data redundancy, and it's the core problem relational databases are specifically designed to solve.

The fix is to split that single table into focused, separate tables — a Customers table (each customer stored exactly once), a Products table (each product stored exactly once), and an Orders table (recording which customer ordered which product, and when) — and then connect them together with relationships, the subject of the rest of this part.