sql - Why SUM(null) is not 0 in Oracle? -

it appreciated explaining internal functionality of sum function in oracle, when encountering null values:
result of

select sum(null) dual; null

but when null value in sequence of values (like sum of null-able column), calculated value of null value 0

select sum(value) ( select case when mod(level , 2) = 0 null else level end value dual connect level <= 10 ) 25

this more interesting when seeing result of

select (1 + null) dual null

as operation null result null (except is null operator).

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some update due comments:

create table odd_table select sum(null) some_name dual;

will result:

create table odd_table (   some_name number )

why some_name column of type number?

sql not treat null values zeros when calculating sum, ignores them:

returns sum of values, or distinct values, in expression. null values ignored.

this makes difference in 1 case - when sequence being totalled not contain numeric items, nulls: if @ least 1 number present, result going numeric.

Addrety