OR: (unknown or true) = true,
    (unknown or false) = unknown    
    (unknown or unknown) = unknown
AND:   
    (true and unknown)  = unknown,     
    (false and unknown)  = false,
    (unknown and unknown) = unknown
NOT:  (not unknown) = unknown

The result of any arithmetic expression involving null is null

按照true(1) > unknown(不知道) > false(0) 来理解