Comment by magicalhippo

I might be missing something as I'm self-taught, but the inner select specifies a set, and you "just" do a simple set membership test?

How it's implemented is as usual up to the database server implementation. Ones I've used creates a temporary table (like it does in so many other cases), and as such EXISTS is usually faster.

But I wouldn't rely on this when moving to another implementation, and use the query planner to see, just as I'd view the assembly output when moving to a new compiler.

Again, I don't have tons of experience, so concrete (counter) examples are welcome.