[ORACLE] 오라클 주민등록번호로 나이 가져오기

/* Formatted on 2012-03-14 오후 5:00:06 (QP5 v5.149.1003.31008) */
SELECT TRUNC (MONTHS_BETWEEN (SYSDATE, TO_DATE (RES, 'YYYYMMDD')) / 12) 
          AS AGE
  FROM (SELECT (CASE
                   WHEN SUBSTR ('8502172111111', 7, 1) IN ('1', '2')
                   THEN
                      '19' || SUBSTR ('8502172111111', 1, 6)
                   WHEN SUBSTR ('8502172111111', 7, 1) IN ('3', '4')
                   THEN
                      '20' || SUBSTR ('8502172111111', 1, 6)
                END)
                  AS RES
          FROM DUAL)