Unnatural Keys – Nature doesn’t come with identifiers
In Sweden, almost everyone is assigned a number at birth, a <i>personnummer</i>, with the format YYMMDD-NNNN (or YYYYMMDDNNNN in computing, to avoid having a Y2K issue every time someone turns 100).<p>A lot of computer systems here seem to treat it as a natural key, which is a terrible idea, because:<p>• Not everyone has one (notably, asylum seekers and temporary residents don't), so if you rely on them, you're excluding those people.<p>• They can change. They won't for 99.9% of people, but not everyone is so lucky. If you use these as your <i>primary</i> key, you're going to create a headache for you and your users for that 0.1% of cases.