Hi,
As I understand it, your first query returned nothing because NULL is
simply "unknown" - and since you can never know what it's equal to,
looking for equality with NULL will never find anything.
Yes, you can basically think of NULLs in SQL as having the same role of zeros in division. They make no sense in principle and must be treated apart. I seem to recall Oracle treating empty strings as NULLs, but this is very dangerous. A NULL means nothing has been assigned, i.e. data is not stated in any way, while an empty string IS an assignment. There is a difference between "nothing" and "this" (no matter what "this" is).
Bèrto