Re: BUG #4057: SUM returns NULL when given no rows

"Reginald Drake" <reggie(dot)drake(at)gmail(dot)com> writes:
> Doing something like "SELECT SUM(some_integer_column) FROM some_table WHERE
> FALSE" gives me NULL, where I would expect 0. Since COUNT does give a
> meaningful value when applied to zero columns, maybe SUM should do the same.

You might expect that, but the SQL spec is entirely clear on the matter:

Case:

a) If the <general set function> COUNT is specified, then the
result is the cardinality of TXA.

b) If AVG, MAX, MIN, or SUM is specified, then

Case:

i) If TXA is empty, then the result is the null value.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ii) If AVG is specified, then the result is the average of the
values in TXA.

iii) If MAX or MIN is specified, then the result is respec-
tively the maximum or minimum value in TXA. These results
are determined using the comparison rules specified in
Subclause 8.2, "<comparison predicate>".

iv) If SUM is specified, then the result is the sum of the
values in TXA. If the sum is not within the range of the
data type of the result, then an exception condition is
raised: data exception-numeric value out of range.

regards, tom lane