[einhverfr]

I disagree with you on NULL. The relational model is a very good way of looking at data in many contexts.

The problem actually is that NULL has several distinct meanings and there is often no real way to differentiate between them other than to disallow all meanings but one, and that is often difficult.

For example you talk about nullable columns, and this is one aspect of NULLs. NULLs may mean missing data. They also are often used to mean the data doesn't apply. This already runs you into ambiguity problems because you can't type in a query that easily distinguishes whether the attribute doesn't apply or is merely unknown. Note that Oracle treats NULL strings as equivalent to empty strings, while PostgreSQL tries to differentiate strings by allowing empty strings which are distinct thus allowing a not-applicable value for character string fields.

Additionally, you would expect the || operator to handle unknown data differently than it does data which doesn't apply. string || not_applicable should equal string. string || unknown should equal unknown.

Now that's only the beginning of the problem. There is a third use of NULLs too, namely as a placeholder for missing rows in outer joins......

If we had three different NULL values some of this problem would be more manageable, but the problem is that as soon as you allow nulls in columns, you can't always easily tell from a query on a well-normalized database what that NULL means without a lot of additional introspection of the representative of the entity set and then you are basically guessing.