UNKNOWN corresponding to the Łukasiewicz I in this article. The addition of IS UNKNOWN to the other operators of SQL's three-valued logic makes the SQL three-valued logic functionally complete, meaning its logical operators can express (in combination) any conceivable three-valued logical function. See also Binary logic (disambiguation) Boolean algebra (structure) Boolean function Digital circuit Four-valued logic Setun – an experimental Russian computer which was based on ternary logic Ternary numeral system (and Balanced ternary) Three-state logic (tri-state buffer) References Further reading , chapters 5-9 Mundici, D. The C*-Algebras of Three-Valued Logic. Logic Colloquium ’88, Proceedings of the Colloquium held in Padova 61–77 (1989). Reichenbach, Hans (1944). Philosophic Foundations of Quantum Mechanics. University of California Press. Dover 1998: Many-valued logic Ternary computers