Why does SUM(...) on an empty recordset return NULL instead of 0?

Question

I understand why null + 1 or (1 + null) returns null: null means "unknown value", and if a value is unknown, its successor is unknown as well. The same is true for most other operations involving null.[*]

However, I don't understand why the following happens:

SELECT SUM(someNotNullableIntegerField) FROM someTable WHERE 1=0

This query returns null. Why? There are no unknown values involved here! The WHERE clause returns zero records, and the sum of an empty set of values is 0.[**] Note that the set is not unknown, it is known to be empty.

I know that I can work around this behaviour by using ISNULL or COALESCE, but I'm trying to understand why this behaviour, which appears counter-intuitive to me, was chosen.

Any insights as to why this makes sense?

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[*] with some notable exceptions such as null OR true, where obviously true is the right result since the unknown value simply does not matter.

[**] just like the product of an empty set of values is 1. Mathematically speaking, if I were to extend $(Z, +)$ to $(Z union {null}, +)$, the obvious choice for the identity element would still be 0, not null, since x + 0 = x but x + null = null.

Answer 1

The ANSI-SQL-Standard defines the result of the SUM of an empty set as NULL. Why they did this, I cannot tell, but at least the behavior should be consistent across all database engines.

Reference: http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt on page 126:

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

         Case:

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

TXA is the operative resultset from the selected column.