Re: 3 value logic. Why is SQL so special?

Cimode wrote:
>
> Not the point. A UNION has nothing to do with a JOIN. They are both
> separate operators applyable on relations. Trying to prove that one is
> derived from another is just
> meaningless, the same way you would try to derive a DIVIDE operator
> with a PLUS operator...This is a dead end.
>

In Codd's algebra that's true but only because it doesn't have a complete set of operators. JOIN is the relational AND operator and UNION is a special case of the corresponding OR operator. Via De Morgan's laws they CAN be defined in terms of each other. NAND or NOR plus REMOVE are the truly primitive operators according to Date and Darwen. Details in TTM(3).

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David Portas