> in school you learn to use natural keys, because e.g. someone's social security number or e-mail address is unique, right? Not sure if this is ironic, but a

> in school you learn to use natural keys, because e.g. someone's social security number or e-mail address is unique, right? Not sure if this is ironic, but a couple of subtleties about uniqueness aren't always conveyed in school. (Not very subtle subtleties given a few minutes' thought, but still...) It's not enough to have single-point-in-time uniqueness, i.e all entities of the type under discussion have unique values of the candiate key now, which is what a uniqueness constraint guarantees. It is also necessary that the value of the candidate key for each entity also does not change for the expected life of the database (or 250 years, whichever is longer--yes, the life of the database, not merely of the entity concerned), so no using people's names, for instance; and as the population of entities changes, values that disappear cannot later reappear (like SSNs of dead people being re-used for a new person). If you don't control key allocation, it's only rarely possible to have ironclad, law-of-physics guarantees about these aspects of keyness.