이렇게 수정해 보셔요~

    SELECT HQ_CODE,

         DECODE(A,1,
             (SELECT MAX(b.BASIC_SEQ) FROM EGRPTEQSS b

                 WHERE b.ANSWER_GRADE = 'Y' AND b.HQ_CODE = a.HQ_CODE),
             (SELECT MAX(b.BASIC_SEQ) FROM EGRPTEQSS b

                 WHERE b.ANSWER_GRADE = 'N' AND b.HQ_CODE = a.HQ_CODE))

               BASIC_SEQ   
FROM (
 SELECT HQ_CODE,
        SIGN(SUM(DECODE(ANSWER_GRADE,'Y',1,0))) A
   FROM EGRPTEQSS
 WHERE ANSWER_GRADE IS NOT NULL
GROUP BY HQ_CODE
) A