If NULL means "I don't know" then IMHO the only sensible and logic correct thing to do is:
SUM(<empty set>) = 0, since that conforms to common-sense algebra. Like SUM(9, 9, 9) = 3x9 = 27 SUM(9, 9) = 2x9 = 18 SUM(9) = 1x9 = 9 SUM(<empty set>) = 0x9 = 0 and certainly not NULL (I don't know). I heard someone say that SUM is a binary operation. It is not, because you can feed it any number of values. It is not the same as "+". Not however that this is not valid for all operations. For example: MIN(<empty set>) = NULL MAX(<empty set>) = NULL AVG(<empty set>) = NULL COUNT(<empty set>) = 0 However: SUM(<set with 1 or more NULL values>) = NULL since 1 of the values is unknown, we can't know the answer Best regards, Frank.
