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;