Uniqueness is Global, Not Local
Everyone agrees that an identifier should be unique. But new database programmers often think that being locally unique is good enough. This leads them to cheerfully use a proprietary auto-increment or IDENTITY column as a PRIMARY KEY instead of as a proper relational key. Aristotles Law of Identity is usually stated as “A is A,” but there was a lot more to it than a formula. Every entity has a nature and follows its nature; every entity is separate and unique from all others in the universe.