A nullable column can always be extracted to a separate table with a non-nullable column and a foreign key. If you left-join this back with the base table, you

A nullable column can always be extracted to a separate table with a non-nullable column and a foreign key. If you left-join this back with the base table, you will get the nulls again for the missing values. So to me it seem nicely symmetrical to use the same kind of NULL value.