The NOT function is used to invert the Boolean value of an expression. It useful for explicitly testing for a false condition.
expression may evaluate to any Boolean result.
The NOT function will return Boolean TRUE if the expression returned a Boolean FALSE. It will return Boolean FALSE of the expression returned a Boolean TRUE.
The NOT function is useful for explicitly testing for the false condition of some test and can clarify the logic of such a test.
In this example the expression MOD(DATE(),7) will return 0 (FALSE) if the day is Sunday and 1 to 6 (TRUE) for the other days. To explicitly test for the day Sunday we need to invert the result of the expression. BY using the NOT function we return a 1 (TRUE) if the day is Sunday and 0 (FALSE) for all other values of the expression.