Oracle Check Constraint

ALTER TABLE CLIENTS
add CONSTRAINT CHK_DISABILITY_INCOME_TYPE_ID CHECK ((IS_DISABLED IS NULL AND DISABILITY_INCOME_TYPE_ID IS NULL) OR (IS_DISABLED = 0 AND DISABILITY_INCOME_TYPE_ID IS NULL) OR (IS_DISABLED = 1));

-- incorrectly succeeds (Why?)
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (null, 1);
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (null, 2);

-- correctly fails
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (0, 1);
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (0, 2);

-- correctly succeeds
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (0, null);
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (1, 1);
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (1, 2);
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (1, null);
INSERT INTO CLIENTS (IS_DISABLED, DISABILITY_INCOME_TYPE_ID) VALUES (null, null);

postgres=> select (null is null and 1 is null);
 ?column?
----------
 f
(1 registro)

postgres=> select (null is null and 1 is null) or (null = 0 and 1 is null);
 ?column?
----------
 f
(1 registro)

postgres=> select (null is null and 1 is null) or (null = 0 and 1 is null) or (null = 1);
 ?column?
----------

(1 registro)

CHECK ((IS_DISABLED IS NULL AND DISABILITY_INCOME_TYPE_ID IS NULL)
    OR (IS_DISABLED IS NOT NULL AND IS_DISABLED = 0 AND DISABILITY_INCOME_TYPE_ID IS NULL)
    OR (IS_DISABLED IS NOT NULL AND IS_DISABLED = 1));

ALTER TABLE CLIENTS ADD CONSTRAINT CHK_1 CHECK (IS_DISABLED = 0 AND DISABILITY_INCOME_TYPE_ID IS NULL)

ALTER TABLE CLIENTS ADD CONSTRAINT CHK_2 CHECK (IS_DISABLED IS NULL AND DISABILITY_INCOME_TYPE_ID IS NULL)

ALTER TABLE CLIENTS ADD CONSTRAINT CHK_3 CHECK (IS_DISABLED = 1)

CHECK
((IS_DISABLED IS NULL AND NVL(DISABILITY_INCOME_TYPE_ID, 0) = 0)
OR (IS_DISABLED = 0 AND NVL(DISABILITY_INCOME_TYPE_ID, 0) = 0)
OR (IS_DISABLED IS NOT NULL AND IS_DISABLED = 1));