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

Arithmetic aggregate functions ignore nulls.

• SUM() ignores them
• AVG() calculates the average as if the null rows didn't exist (nulls don't count in the total or the divisor)