多层查询的问题....

Question

SELECT 6,(SELECT decode(month(Sysdate),1,sum1.s-sum2.s,sum1.s-sum2.s+sum3.s-sum4.s)
          FROM     
              (SELECT sum(lsi.ONE_MONTH_AMT) s FROM XX_LEDGER_STAT_IFS lsi
               WHERE 
                    EXISTS 
                         (SELECT 1 FROM XX_PA_GEN_FTP_UNIT_ID pg
                             WHERE pg.INSTRUMENT_TYPE_CD = 560 
                             AND pg.GL_ACCT_ID = lsi.GL_ACCT_ID)
                    AND  substr(lsi.GL_ACCT_ID,1,1) = '4'
               GROUP BY 
                    lsi.ISOCRNCYCD,
                    lsi.ORG_ID,  
                    lsi.DEPT_ID, 
                    lsi.GL_ACCT_ID) sum1,
              (SELECT sum(lsi.ONE_MONTH_AMT) s FROM XX_LEDGER_STAT_IFS lsi
               WHERE 
                    EXISTS 
                         (SELECT 1 FROM XX_PA_GEN_FTP_UNIT_ID pg
                             WHERE pg.INSTRUMENT_TYPE_CD = 560 
                             AND pg.GL_ACCT_ID = lsi.GL_ACCT_ID)
                    AND  substr(lsi.GL_ACCT_ID,1,1) = '5'
               GROUP BY 
                    lsi.ISOCRNCYCD,
                    lsi.ORG_ID,  
                    lsi.DEPT_ID, 
                    lsi.GL_ACCT_ID) sum2,
              (SELECT sum(ytd_n) s FROM LEDGER_STAT ls
               WHERE 
                    EXISTS 
                         (SELECT 1 FROM XX_PA_GEN_FTP_UNIT_ID pg
                             WHERE pg.INSTRUMENT_TYPE_CD = 560 
                             AND pg.GL_ACCT_ID = ls.GL_ACCOUNT_ID)
                    AND  substr(ls.GL_ACCOUNT_ID,1,1) = '4'
                    AND  ls.YEAR_S = xx.YEAR_S
               GROUP BY 
                    ls.ISO_CURRENCY_CD,
                    ls.ORG_UNIT_ID,  
                    ls.DEPT_ID, 
                    ls.GL_ACCOUNT_ID) sum3,
              (SELECT sum(ytd_n) s FROM LEDGER_STAT ls
               WHERE 
                    EXISTS 
                         (SELECT 1 FROM XX_PA_GEN_FTP_UNIT_ID pg
                             WHERE pg.INSTRUMENT_TYPE_CD = 560 
                             AND pg.GL_ACCT_ID = ls.GL_ACCOUNT_ID)
                    AND  substr(ls.GL_ACCOUNT_ID,1,1) = '5'
                    AND  ls.YEAR_S = xx.YEAR_S
               GROUP BY 
                    ls.ISO_CURRENCY_CD,
                    ls.ORG_UNIT_ID,  
                    ls.DEPT_ID, 
                    ls.GL_ACCOUNT_ID) sum4),
              xx.ORG_ID      
      FROM XX_LEDGER_STAT_IFS xx;                报错:
PROCEDURE BOCOFSA.PROC_FPF_WBSYYJ 编译错误错误：PL/SQL: ORA-00904: "XX"."YEAR_S": invalid identifier
行：103
文本：AND  ls.YEAR_S = xx.YEAR_S是什么原因?子查询的子查询就不能引用父查询的表了?