Yea, I ask the employer/manager query problem sometimes. I'm not looking for the a right answer, but rather how they think through the problem and how they think about depth bounds. Occasionally I'll have a candidate who can throw out an oracle "connect by," but it is absolutely not what I'm looking for. (Even though that's probably the "most correct" answer).
I'd say it's unlikely you know about self joins but don't know at least one syntax for it just because of how we traditionally teach SQL to others. This is the know-it-or-not part: SQL is rarely taught without syntax and syntax is used to illustrate the theories. This is in contrast to regular programming where people just pick up pseudo code eventually.
If you had a pseudo code for sets, joins, etc, it'd probably be using more mathematical symbols.
In the case of percent change in MAU, you don’t need to know how to do a self join, exactly. But rather to identify that you would want to do a self join and on what conditions would be the key