There's also a lot of essential complexity there. SQL provides, in essence, a generic interface for entering and analyzing data. Imagine the number of ways to structure and analyze data. Now square that number to get the number of two tests for how two basic features of the language interact with each other. And that's not even near full test coverage.

Your point about essential complexity is absolutely correct, but your faux mathematical analysis is totally not a legit way to analyze the complexity of something or determine test coverage. I feel like as programmers we should be comfortable making sensible statements without making up shady pseudo-math to sound convincing.

It's abundantly clear that I'm not making a precise computation here. My argument is that tests don't scale linearly with the number of features because interactions between features need to be tested as well.