Normalisation in practice

Every fact in exactly one place

Normalisation is one idea applied repeatedly: store each fact once. Storing a customer address on every order means an address change updates some rows and not others, and now the data disagrees with itself.

A well-normalised schema makes writes safe and joins routine. Modern databases join efficiently; the fear of joins is usually inherited rather than measured.

Denormalising is legitimate when you have measured a problem you cannot solve otherwise. It is a trade you make consciously, and it comes with the obligation to keep the copies in sync.