select 
    S.DEPT_CODE as DEPT_CODE
    ,DEPT_NAME
from
    TBL_SALES as S
    left outer join TBL_DEPT as D
        on S.DEPT_CODE=D.DEPT_CODE
union
select
    O.DEPT_CODE as DEPT_CODE
    ,DEPT_NAME
from
    TBL_ORDERS as O
    left outer join TBL_DEPT as D
        on O.DEPT_CODE=D.DEPT_CODE;