Both IS NULL
and COALESCE
are ANSI standard and available in almost all reasonable databases. The construct that you want, I think, is:
where region IS NULL
This is standard syntax.
To have COALESCE
work like IS NULL
requires a value that you know is not in the data:
where coalesce(region, '<null>') <> '<null>'
However, you would need different values for dates and numbers.