You think your surrogate key will save you? It will not. The world has an external reality that needs to be reflected in your database. If the unique identifier for your object — VIN, CUSIP, whatever — if it changes, the world will henceforth refer to it by both. You will need to track both. Adding a synthetic key only means you have to track all three. Plus you have to generate a meaningless number, which is actually a choke point in your data throughput.
The natural key forces you to think about what makes the row unique. What identifies it. Sometimes, it makes you go back to the SME and ask them what they mean. Sometimes it makes you reconsider time: it’s unique now, but does it change over time, and does the database need to capture that? In short, what are the boundaries of the Closed World Assumption? You need to know that too, to answer any "not exists" question.
To use our professor’s car’s example, we actually do not know the database design. It could well be that the original identifier remained the primary key, and the "new id" is entered as an alias. The ID is unique in the Car table, identifying the vehicle, and is not in the CarAlias table, where the aliases are unique.
Oh, you say, but what if the bad old ID gets reused? Good question. Better question: how will the surrogate key protect you? It will not. The reused ID will be used to query the system. Without some distinguishing feature, perhaps date, it will serve up duplicates. The problem has to be handled, and the surrogate key is no defense.
Model your data on the real world. Do not depend on spherical horses.
The natural key forces you to think about what makes the row unique. What identifies it. Sometimes, it makes you go back to the SME and ask them what they mean. Sometimes it makes you reconsider time: it’s unique now, but does it change over time, and does the database need to capture that? In short, what are the boundaries of the Closed World Assumption? You need to know that too, to answer any "not exists" question.
To use our professor’s car’s example, we actually do not know the database design. It could well be that the original identifier remained the primary key, and the "new id" is entered as an alias. The ID is unique in the Car table, identifying the vehicle, and is not in the CarAlias table, where the aliases are unique.
Oh, you say, but what if the bad old ID gets reused? Good question. Better question: how will the surrogate key protect you? It will not. The reused ID will be used to query the system. Without some distinguishing feature, perhaps date, it will serve up duplicates. The problem has to be handled, and the surrogate key is no defense.
Model your data on the real world. Do not depend on spherical horses.