Why SUM(null) is not 0 in Oracle?

Question

It would be appreciated explaining the internal functionality of SUM function in Oracle, when encountering null values:
The result of

select sum(null)          


        

Answer 1

SQL does not treat NULL values as zeros when calculating SUM, it ignores them:

Returns the sum of all the values, or only the DISTINCT values, in the expression. Null values are ignored.

This makes a difference only in one case - when the sequence being totalled up does not contain numeric items, only NULLs: if at least one number is present, the result is going to be numeric.