Оператор

CREATE TABLE TBOOL (ID INT, BVAL BOOLEAN);
COMMIT;

INSERT INTO TBOOL VALUES (1, TRUE);
INSERT INTO TBOOL VALUES (2, 2 = 4);
INSERT INTO TBOOL VALUES (3, NULL = 1);
COMMIT;

SELECT * FROM TBOOL

ID           BVAL
============ =======
1            <true>
2            <false>
3            <null>

SELECT * FROM TBOOL WHERE BVAL

ID           BVAL
============ =======
1            <true>

SELECT * FROM TBOOL WHERE BVAL IS FALSE

ID           BVAL
============ =======
2            <false>

SELECT * FROM TBOOL WHERE BVAL IS UNKNOWN

ID           BVAL
============ =======
3            <null>

SELECT ID, BVAL, BVAL AND ID < 2
FROM TBOOL

ID           BVAL
============ ======= =======
1            <true> <true>
2            <false> <false>
3            <null> <false>

DECLARE VARIABLE VAR1 BOOLEAN = TRUE;

-- Допустимый синтаксис, но как и сравнение
-- с NULL, никогда не вернёт ни одной записи
SELECT * FROM TBOOL WHERE BVAL = UNKNOWN
SELECT * FROM TBOOL WHERE BVAL <> UNKNOWN