select
    OD.DEPT_CODE as DEPT_CODE
    ,DP.DEPT_NAME 
from
    TBL_ORDERS as OD 
LEFT OUTER JOIN TBL_DEPT as DP 
        ON DP.DEPT_CODE = OD.DEPT_CODE 

union

select
    SA.DEPT_CODE as DEPT_CODE
    , DP.DEPT_NAME 
from
    TBL_SALES as SA 
    LEFT OUTER JOIN TBL_DEPT as DP 
        ON DP.DEPT_CODE = SA.DEPT_CODE 
ORDER BY
    DEPT_CODE asc;