Recently there was some online complaints about social security numbers (SSNs) in the US being duplicated and re-used by individuals. This is really political gamesmanship, so ignore the political part. Just know that social security numbers appear to be one of the contenders used in many data models.

I found a good piece about how SSNs aren't unique, and have a mess of problems. Despite this, many people seem to want to use SSNs as a primary or alternate key in their database systems. They also aren't well secured in many systems, even though we should consider this sensitive PII data.

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