One of the things that baffles me with SQL is how some things make complete sense, but when you apply the same logic, it just doesn’t work. For example:

select *
from payments
where amount > 20;

This works and it makes sense that it works. Then you try this:

select *
from payments
where avg(amount);

And you get a grouping error. Which makes no sense. Why shouldn’t you be able to make that work? Instead, you have to write it as a subquery.