USING 절에 dual 쓰지 말고 SELECT 문을 쓰세요.

MERGE INTO t_schedule a
USING
(
SELECT #{dept}      dept
     , #{dataNum}   data_num
     , #{gnName}    gn_name
     , MIN(fcty_id) sch_seq3
     , MIN(inst_s)  network_con
  FROM t_fcty
) b
ON (a.dept = b.dept AND a.data_num = b.data_num)
WHEN MATCHED THEN
UPDATE
   SET a.gn_name     = b.gn_name
     , a.sch_seq3    = b.sch_seq3
     , a.network_con = b.network_con
WHEN NOT MATCHED THEN
INSERT ( dept
       , data_num
       , gn_name
       , sch_seq3
       , network_con
       )
VALUES ( b.dept
       , b.data_num
       , b.gn_name
       , b.sch_seq3
       , b.network_con
       )
;

올려 주신 쿼리 보고 그대로 작성한 쿼리입니다.
다만, 올려주신 쿼리가 완성형이 아닌 듯 한 느낌이 드네요.
업무 요건에 맞게 쿼리를 완성하셔야 할 것 같습니다.

UPDATE T_SCHEDULE A
			 		SET
						 NETWORK_CON =	B.INST_S,
						IP = B.PUBLIC_IP,
						MODEM_CNT = B.MODEM_CNT,
						INSTR_CNT = B.METER_CNT,
						M_COMPANY = B.M_COMPANY,
						FROM(SELECT INST_S,PUBLIC_IP,MODEM_CNT,METER_CNT,M_COMPANY FROM T_FCTY) AS B
		WHERE 	
			 B.DCU_ID = A.DCU_ID
			AND B.N_DEPT1 = A.N_DEPT1
			AND B.N_DEPT2 = A.N_DEPT2
			AND B.DEPT1= A.DEPT1
			AND B.DEPT2= A.DEPT2	
			AND B.BUSI_DIV= A.BUSI_DIV
			AND B.DCU_ID =#{dcuId}
			AND B.N_DEPT1 = #{nDept1}
			AND B.N_DEPT2 = #{nDept2}
			AND B.DEPT1= #{dept1}
			AND B.DEPT2= #{dept2}	
			AND B.BUSI_DIV= #{busiDiv}	

답변감사합니다. 혹시 SELECT UPDATE 문도 알려주실수있을까요 인터넷에서는 저렇게 하라는데 ㅠ 안돼네요

각자 하나에대한 서브쿼리는 너무 많이돌리니깐 셀렉트문  하나에 T_SCHEDULE  넣고싶은데 ㅠ방법이있을까요

UPDATE t_schedule a
   SET ( sch_seq3
       , network_con
       , ip
       , modem_cnt
       , instr_cnt
       , m_company
       , gaetong_id
       , gaetong_date
       )
     = (SELECT b.id
             , b.inst_s
             , b.public_ip
             , b.modem_cnt
             , b.meter_cnt
             , b.m_company
             , b.reg_id
             , b.reg_date
          FROM t_fcty b
         WHERE b.dcu_id   = a.dcu_id
           AND b.n_dept1  = a.n_dept1
           AND b.n_dept2  = a.n_dept2
           AND b.dept1    = a.dept1
           AND b.dept2    = a.dept2
           AND b.busi_div = a.busi_div
        )
 WHERE a.dcu_id   = #{dcuId}
   AND a.n_dept1  = #{nDept1}
   AND a.n_dept2  = #{nDept2}
   AND a.dept1    = #{dept1}
   AND a.dept2    = #{dept2}
   AND a.busi_div = #{busiDiv}
   AND EXISTS
       (SELECT 1
          FROM t_fcty b
         WHERE b.dcu_id   = a.dcu_id
           AND b.n_dept1  = a.n_dept1
           AND b.n_dept2  = a.n_dept2
           AND b.dept1    = a.dept1
           AND b.dept2    = a.dept2
           AND b.busi_div = a.busi_div
        )
;