> So then, if there are no input rows at all (if no rows > match the WHERE clause) then SUM returns 0. (This makes > sense because if you say: > > SELECT sum(amt) FROM sales WHERE month='october'; > > and you didn't sell anything in October, you want an > answer of 0, not NULL.) Or if *some* of the entries are
You (and I) may want an answer of 0, but I unfortunately believe NULL is the correct answer. The answer isn't 0 - the answer is "there is no answer", because there were no inputs. If that translates to 0 in reality that's up to you to interpret. :) I think the set-theoretical explanation is that an operation on a NULL set is NULL. COUNT seems to be an exception there, though... Hrmph. > The more I learn about NULLs in SQL the less sense they > make... I have never intentionally declared a column that allowed NULLs. :) -Tom
