Posted by --CELKO-- on 04/08/07 14:00

>> It seems like dropping UNKNOWN would leave a sensible set of rules:

and | T N F or | T N F not |
----+------ ---+------ ----+--
T | T N F T | T T T T | F
N | N N F N | T N N N | N
F | F F F F | T N F F | T

Am I overlooking anything? <<

The NULL propagation rule.

and | T N F or | T N F not |
----+------ ---+------ ----+--
T | T N F T | T N T T | F
N | N N N N | N N N N | N
F | F N F F | T N F F | T

This means that TRUE OR NULL = NULL, etc. and you can now prove that
TRUE = FALSE. The UNKNOWN logical value does not have this behavior
and that is why we have it.

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