PaulB-TheOneAndOnly (1/21/2010)
I can see no conflict between data normalization and accepting null values in non-key columns - as all rdbms out there allow and enforce.
All the conventional Normal Forms apply exclusively to relations with values, never nulls. In fact normalization predates the invention of SQL and nulls. So you can't say that any table with a null represents a relation in Normal Form.
If you allow an attribute to be null under normalization, you either have to ignore dependencies involving that attribute or you must make an informal (and incorrect) assumption that null can always be treated like a proper value. Formally speaking nulls just can't work with normalization. In particular, there is no clear answer as to what nonloss decomposition or join dependency means for tables that have nulls in them. So no table with a null is truly normalized. Since it is always possible to decompose further and eliminate nulls they are not formally necessary either - you can always create a truly normalized design without them.