If you want to compare values against NULL as conditions, you should use the "IS NULL" or "IS NOT NULL" operator. Do not use "=" or "<>" against NULL. The sample script below shows you some good examples:

SELECT 'A' IS NULL FROM DUAL;

-- Error: Boolean is not data type.

-- Boolean can only be used as conditions

SELECT CASE WHEN 'A' IS NULL THEN 'TRUE' ELSE 'FALSE' END

FROM DUAL;

FALSE

SELECT CASE WHEN '' IS NULL THEN 'TRUE' ELSE 'FALSE' END

FROM DUAL;

TRUE

SELECT CASE WHEN 0 IS NULL THEN 'TRUE' ELSE 'FALSE' END

FROM DUAL;

FALSE

SELECT CASE WHEN NULL IS NULL THEN 'TRUE' ELSE 'FALSE' END

FROM DUAL;

TRUE

SELECT CASE WHEN 'A' = NULL THEN 'TRUE' ELSE 'FALSE' END

FROM DUAL;

-- Do not use "="

FALSE

SELECT CASE WHEN 'A' <> NULL THEN 'TRUE' ELSE 'FALSE' END

FROM DUAL;

-- Do not use "<>"

FALSE

SELECT CASE WHEN NULL = NULL THEN 'TRUE' ELSE 'FALSE' END

FROM DUAL;

-- Do not use "="

FALSE