Why NULL values are mapped as 0 in Fact tables?

Question

What is the reason that in measure fields in fact tables (dimensionally modeled data warehouses) NULL values are usually mapped as 0?

Answer 1

It depends upon what you're modeling, but in general it's to avoid complications with performing aggregates. And in many scenarios it makes sense to treat NULL as 0 for those purposes.

For example, a customer with NULL orders for a given period of time. Or a sales person with NULL sales revenue (shame on him!).