Oracle-受理编号取最大值生成有关问题sql

SELECT 'JGA'||TO_CHAR(SYSDATE,'YYMM')||LPAD(SUBSTR(MAX_T_NO,8,4)+1,4,'0') NEXT_TRANSTER_ITEMNO FROM
          (
          SELECT    'JGA'||  NVL(  max(cast (substr(P.TRANSTER_ITEMNO,4) as number ))   , TO_CHAR(SYSDATE,'YYMM')||'000' ) MAX_T_NO
            FROM LISTING_PROJECT P
           WHERE P.TRANSTER_ITEMNO LIKE 'JGA' || TO_CHAR(SYSDATE, 'YYMM') || '%'
          ) V