Keys and relationships

Keys are how rows find each other

A surrogate key is stable and meaningless, which is exactly what you want for identity. Natural keys change — email addresses, order numbers, country codes — and a changing primary key propagates through every referencing row.

Foreign keys are cheap insurance. Without them, orphaned rows accumulate quietly and cleanup scripts become a permanent fixture.

Many-to-many needs a join table. Once it exists it usually wants its own columns anyway — when the relationship was created, by whom, in what role.