You could, but that introduces a lot of complexity that would be saved by just giving the customer a synthetic ID that uniquely and stably identifies them in yo

You could, but that introduces a lot of complexity that would be saved by just giving the customer a synthetic ID that uniquely and stably identifies them in your system. The grandparent's proposal basically turns the first-entered CPR into a meaningless ID field that should not generally be used in a piece of business logic (unless for some reason you need to display "first CPR they entered"). Once you've declared that you should look elsewhere if you actually need the CPR, why did we even bother using it as the primary key?