NULL value

Predicates in relational databases use three-valued logic (3VL), where there are three truth values: true, false, and unknown aka NULL. In a relational database, the third value, NULL, can be interpreted in many ways, such as unknown data, missing data, not applicable, or will be loaded later. The three-valued logic is used to remove ambiguity. For example, no two NULL values are equal.

The following table shows logical OR / AND truth operator; note that these operators are commutative, that is, A AND B = B AND A: