NOT Operator

If you preface a condition with the keyword NOT, the test is TRUE only if the condition that NOT qualifies is FALSE. If the condition that NOT qualifies has a NULL or an UNKNOWN value, the NOT operator has no effect.

The following truth table shows the effect of NOT with 3–valued Boolean operands. Here T represents a TRUE condition, F represents a FALSE condition, and a question mark (?) represents an UNKNOWN condition. (An UNKNOWN value can occur when an operand is NULL).
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The left column shows the value of the operand of the NOT operator, and the right column shows the returned value after NOT is applied to the operand.