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
    SL.DEPT_CODE AS DEPT_CODE,
    DP.DEPT_NAME
FROM
    TBL_SALES AS SL
    LEFT OUTER JOIN TBL_DEPT AS DP
        ON DP.DEPT_CODE = SL.DEPT_CODE
ORDER BY
    DEPT_CODE;