Booleans and UNKNOWN

TRUE and FALSE are concrete booleans. UNKNOWN represents a missing truth value. AND, OR, XOR and NOT propagate unknown according to three-valued logic.

RETURN NOT UNKNOWN AS unknown, FALSE AND UNKNOWN AS false_value,
       TRUE OR UNKNOWN AS true_value;

Truth predicates include IS [NOT] TRUE, IS [NOT] FALSE and IS [NOT] UNKNOWN. They test the truth category, rather than using ordinary equality with null.

RETURN UNKNOWN IS UNKNOWN AS unknown,
       UNKNOWN IS NOT FALSE AS not_false;

WHERE and FILTER retain only true rows. Declared BOOLEAN session initializers also accept NOT UNKNOWN; the result is a null value subject to the declaration’s nullability constraint.