It works until it doesn't :). A lot depends on how well they're enforced. SSNs are a go-to example, but in other countries which have actual ID numbers for citizens[0], duplicates do happen every now and then because of e.g. clerical errors. Real world is messy, and something can be trivially fucked up, it will be.

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[0] - as opposed to the way US handles the SSN, where the number was explicitly forbidden from being used for identification, but people use it for it anyway.

There are not only duplicates, but sometimes the documents can be reissued with another number or ID, sometimes there are people without the document that your system expects everybody to have.

Here in Brazil, we have the CPF (somewhat equivalent to US SSN), a "unique" number assigned by the government for identification. It's used by almost every business I have ever know as a key. Not always primary key, but certainly people treat it as unique.

Except for the fact they are not unique, and there have been cases where people from different states have the same number because each state generated this "random" number, but didn't actually check with each other if it's really unique. It is legally enforced, no two people should ever have the same, but they do.

The only way to have uniqueness is if you know the algorithm AND can control that it behaves as expected. Anything else is just kidding yourself.

Even that is not a forever guarantee. Laws can and do change.

And people act outside the bounds of the law. SSN seems like a good natural key candidate, until you discover some of the people your business interacts with are illegal aliens.

Or legal aliens, like tourists. Or foreigners, who aren't even in the country. Or really anyone else who doesn't work in the country, and doesn't have to report taxes.

Ironically, many illegal immigrants actually do report taxes. In which case they use ITIN, which is compatible to SSN format-wise (###-##-####)... but it is not unique over time.

Definitely true, but at a certain point you need to draw a line. If these things were to change, the entire product would have to change, not just what I use as my keys, so I think natural keys are a safe bet in my case.

> Thankfully mine are legally enforced, so it works well

What, you think laws never change?