It's noteworthy that SQL's 3VL follows Kleene's K3 logic (or, equivalently, Łukasiewicz' Ł3 logic) and not Priest's P3 logic. That is, in K3 and Ł3, an unknown (or indeterminate/null) value resolves to neither true nor false; whereas in K3, an unknown (or indeterminate/null) value resolves to both true and false.

There are some interesting ramifications of these properties, including a lack of tautologies in K3 and Ł3.