It's easy and even expected to have a requirement. There's a requirement that US SSNs are unique too. A requirement doesn't cut it. The requirement is going to fail sooner or later and what then? How much did the manufacturers you worked with indemnify you against the consequences of non-unique serial numbers?

That's what we're getting at here. When it breaks somebody has to fix it. Or, you could design it not to break.

It's a man's laws versus nature's laws problem. Did you know you can pick up the soccer ball, and just carry it over the goal line? Yeah, that's "against the rules" but you can do it anyway, being against the rules doesn't make it impossible.

Now, contrariwise you can't teleport the ball - doesn't matter whether the rules allow it, can't do it anyway. Nature's law are facts. Design your database to rely on facts, not man's laws as much as possible.

You buy the other F-150 sn 1234. You take your truck in for service. They refuse to service salvage title vehicles under warranty. You explain you don’t have a salvage title. They blame “the system”. Your insurance company fares no better.

Or, Ford could put a unique constraint on serial number and catch the problem in the factory.

It’s not clear to me how the internal_db_id did anything except propagate an error until it was way too late to fix.

The entire point of properly modeling data is to codify and enforce the “rules”.

> Nature's law are facts. Design your database to rely on facts, not man's laws as much as possible.

Exactly. Natural keys can, along with normalization and constraints, stop you from picking up the ball and walking into the goal. In other words, natural keys define the nature of your system.

First of all it'll get picked up by the people who set up the serial number printer. They don't just set it to a random number - the factory will have a procedure to make sure the number is set right.

Then it'll get picked up by the end-of-line testing, as test reports are filed by serial number.

Then it'll get picked up when the first part comes off the production line and gets checked by the factory's most conscientious testers and engineers.

Then it'll get picked up at goods inbound, when they count the items and check them in, and find the system says some of them are used, or there's a discrepancy in the number of items.

Then it'll get picked up at acceptance testing, when they find the shipment's test records aren't in order.

Then it'll get picked up by accounts payable, when the manufacturer bills for parts they've already been paid for.

Then depending on the numbers and costs involved, you either send the bad units back for relabelling or trash them, in either case at the manufacturer's expense.

Printer ran out of toner and nobody noticed until the next shift.

> Then it'll get picked up by the end-of-line testing, as test reports are filed by serial number.

Nah, test report operator was out sick that day and product needed to be pushed anyway. Manager assumed they're all good and overrode tests.

> Then it'll get picked up when the first part comes off the production line and gets checked by the factory's most conscientious testers and engineers.

Those testers and engineers don't have all day to test every single product. They only tested the first 100.

> Then it'll get picked up at goods inbound, when they count the items and check them in, and find the system says some of them are used, or there's a discrepancy in the number of items.

Counted by total weight divided by individual weight.

> Then it'll get picked up at acceptance testing, when they find the shipment's test records aren't in order.

Counted total number of UPCs.

> Then it'll get picked up by accounts payable, when the manufacturer bills for parts they've already been paid for.

Accounts payable sees a box, of 1000 parts and they're labeled and the top 100 appear genuine... pay it.

> Then depending on the numbers and costs involved, you either send the bad units back for relabelling or trash them, in either case at the manufacturer's expense.

Manufacturer thinks you're trying to defraud them and won't authorize a return without a serial number.

The whole point of a serial number is to be able to track an item through your supply chain

If it can get through your supply chain without anyone ever reading the serial number, why are you putting a serial number on it at all? Decoration?

My point is that even though there are multiple points where the serial number "should" be checked, there are also multiple points where that could fail for one reason or another.