I am testing on Visual FoxPro 6, no service pack:

? NULL and .T. ==> NULL
? NULL and .F. ==> .F.

Do you get different results in another version?


Sorry, A my error.

Docs Behavior of Null Values in Logical Expressions explain this, and this is correct.

Because AND and OR have symmetrical proprerty, => XRY = YRX,
the cut of VFP logic evaluation can return not correct values
if the first operand is NULL;
then a NULL value it cannot cut the logic

? .F.    AND m.X <=== return .F. for every X, and X is not evaluated
? .NULL. AND m.X <=== must return .F. if m.X, then m.X must to be evaluated
? .T.    OR  m.X  <=== return .T. for every X, and X is not evaluated
? .NULL. OR  m.X <=== must return .T. if m.X, then m.X must to be evaluated