I agree, I should have written that it helps to think in sets rather than tables. SQL implementors do a lot of fantastic shit to get our queries to run in a reasonable time, and SQL is not one of those langages that are like "understand this one core principle, and you get it" like lisp, prolog, TCL, Smalltalk, Forth etc. Have a look at the query plan if your DBMS can output it, it's not always easy to guess what it will spit out. In the case of

    (select a from nums) order by b;

my best guess is that the parser helpfully strips out the parens as because the "order by" needs to be a part of a table expression to make sense. This expression is a syntax error in SQLite by the way.