SQL doesn't have this "belief". SQL has the behavior that is most useful for querying and joining when some columns have unset values, or foreign key relations

SQL doesn't have this "belief". SQL has the behavior that is most useful for querying and joining when some columns have unset values, or foreign key relations that are unset. I understand your position, but your idealism would be strictly worse in real life. Can you imagine if an optional foreign key relation immediately made all joins fail because one was unset? Or if a join between two optional values made all NULL values join together? The SQL NULL behavior also allows UNIQUE nullable columns to have more than one instance that is NULL, so you can have uniqueness over a column that may also be unset. If you're a purist or an idealist, SQL NULL might be a mess, but pragmatically, it's the only behavior that is sane for SQL as-is.