Common logical operators are AND, OR, NOT, with three result values TRUE, FALSE, and NULL, where NULL represents an unknown value.
a | b | a AND b | a OR b | NOT a |
TRUE | TRUE | TRUE | TRUE | FALSE |
TRUE | FALSE | FALSE | TRUE | FALSE |
TRUE | NULL | NULL | TRUE | FALSE |
FALSE | FALSE | FALSE | FALSE | TRUE |
FALSE | NULL | FALSE | NULL | TRUE |
NULL | NULL | NULL | NULL | NULL |