Erik Darling reminds us that even good things can go bad:
Look, I really like EXISTS and NOT EXISTS. I do. They solve a lot of problems.
This post isn’t a criticism of them at all, nor do I want you to stop using them. I would encourage you to use them more, probably.
But there’s some stuff you need to be aware of when you use them, whether it’s in control-flow logic, or in queries.
But do read on to see a specific type of issue you can run into with a left semi join.