OK folks, just so you know, I've been reading and re-reading these replies, especially Jeff Moden's to make sure everything is completely clear to me. Then, after that if I still have questions I just can't figure out I'll post. It's really wonderful you are here helping me.

At this point, the recurrent question lurking in my mind is 'why do we have to count from zero' instead of '1' referring to 'SELECT N = ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) -1

'...but sometimes these things become clear after the 5th reading etc. (think tortoise and the hare)