NULL is the absence of value.
Any aggregation operation on a set where ALL the elements of the set
are "absences of values" should return an "absence of value." If ALL
the elements are NOT NULL then the aggregation operation should be
applied to the elements that have a "presence of value."
The only exception should be COUNT as it would return the number of
elements in the set without peeking in at the values of the members of
the set, hence never even encountering the absence or presence of
values of them.
--
Puneet Kishor
