In simple expression, it's natural to turn to null for insensible operations, eg. (1 + null => null).

By design, null is a special value included in all types. And in primitive operations, usually the result will be null if any operand is null except IS NULL.

However, there're more special handling required to make SQL useful.

In aggregation, we don't want to NULLify the whole state on first met of a null value. So maybe, for sum/cumsum, we can treat nulls as 0s.

Still not sure if any other special cases in aggregation functions.

Also, in sorting, should null be min or max.