SELECT
    DISTINCT ( CASE
        WHEN TD.DEPT_CODE IS NULL THEN T.DEPT_CODE
        ELSE TD.DEPT_CODE
        END) AS DEPT_CODE
    , TD.DEPT_NAME AS DEPT_NAME

FROM
    TBL_DEPT AS TD
    FULL OUTER JOIN
    (SELECT
        DISTINCT(
            CASE
                WHEN TA.DEPT_CODE IS NULL THEN TS.DEPT_CODE
                ELSE TA.DEPT_CODE
                END) AS DEPT_CODE
    FROM
        TBL_ORDERS AS TA
        FULL OUTER JOIN
            TBL_SALES AS TS
            ON TA.DEPT_CODE = TS.DEPT_CODE
    ) AS T
    ON TD.DEPT_CODE = T.DEPT_CODE
ORDER BY
    DEPT_CODE ASC