SET PSTN_ORGN =

     (SELECT max(A.VAL1) AS PSTN_ORGN

      FROM T_DESM_USER_TEMP A

      WHERE A.USER_ID = B.USER_ID   

     )

FROM T_DESM_USER B

    WHERE   B.USER_ID  in ( select A.USERID

     FROM T_DESM_USER_TEMP A

2.1. T_DESM_USER 와 T_DESM_USER_TEMP 가 1대1이거나

      아무값이나 update 해도 되는경우

UPDATE T_DESM_USER

    SET PSTN_ORGN = A.VAL1    

FROM T_DESM_USER B, USER_TEMP A

    WHERE   A.USER_ID = B.USER_ID