You need to move the conditions on the second table into the on
clause:
SELECT q.ID, q.CORRECT_ANSWER, qa.RESPONSE
FROM QUESTIONS q LEFT JOIN
QUESTIONS_ATTEMPTED qa
ON qa.QUESTION_ID = q.ID and qa.TEST_SESSION_ID = 235
WHERE q.TEST_ID = 1 ;
Otherwise, the value is NULL
on the non-matched rows and this fails the comparison.