Natural keys are also a nightmare to deal with when you have to do complex joins. They are not guaranteed to be a single column value and can lead to joining across a half dozen columns when you need data from multiple tables.
Usernames are actually a very good example of something that looks like an invariant when in actuality is anything but. On top of that, they are notoriously inconsistent between systems so you can't rely on usera in one system being usera in another even when you control both systems.
Even if you do have a nice single column natural key, what happens when a business requirement changes and there's suddenly a completely new coding system? Or perhaps your coding system needs a new distinction, do you rely on hand or create a new column? How many tables to you have to touch to change to accommodate the new data? How much code is relying your tables be structured just so?
Natural keys look appealing on face, but they can all too easily lead to a big mess down the road. The larger and more complex the system, the more external systems that you have to integrate, the bigger the mess becomes.
Usernames are actually a very good example of something that looks like an invariant when in actuality is anything but. On top of that, they are notoriously inconsistent between systems so you can't rely on usera in one system being usera in another even when you control both systems.
Even if you do have a nice single column natural key, what happens when a business requirement changes and there's suddenly a completely new coding system? Or perhaps your coding system needs a new distinction, do you rely on hand or create a new column? How many tables to you have to touch to change to accommodate the new data? How much code is relying your tables be structured just so?
Natural keys look appealing on face, but they can all too easily lead to a big mess down the road. The larger and more complex the system, the more external systems that you have to integrate, the bigger the mess becomes.