Oracle merge into ：无法更新 ON 子句中引

MERGE INTO ZHXX T1
            USING ACCOUNTS_TEMP T
            ON 
           ( T1.ACCNO = T.ACCNO AND   (T.STATE='9' OR T.STATE='10'  ) AND  (T1.STATE IS NULL) )
            WHEN MATCHED THEN
                UPDATE
                    SET
                        T1.state = T.state,
                        T1.memo = T.memo

改为

MERGE INTO ZHXX T1
            USING ACCOUNTS_TEMP T
            ON ( T1.ACCNO = T.ACCNO AND (T.STATE='9' OR T.STATE='10' ) ) 
            WHEN MATCHED THEN
                UPDATE
                    SET
              T1.MEMO = DECODE(T1.MEMO, NULL, T.MEMO, T1.MEMO),
                     T1.STATE = DECODE(T1.STATE, NULL,T.STATE,T1.STATE)
                                         -- DECODE(变量S，如果S=NULL,S=1,否则S=2)