I fundamentally don’t understand SQL JOIN clauses.
That’s not to say I can’t read and parse them, I just don’t understand them well enough to be able to write them and be confident of exactly what’s happening. In my brain, this means I fail some kind of test. I think that being able to naturally understand and write JOIN clauses is probably what marks you out as a Computer Scientist, instead of some pretender like me.
Perhaps JOINs are some form of Computer Science Turing test? Although whether being able to write JOINs means you pass or fail the test I’ll leave up to you.