SELECT
    OD.DEPT_CODE AS DEPT_CODE, 
    DP.DEPT_NAME
FROM
    TBL_ORDERS AS OD
    LEFT JOIN TBL_DEPT AS DP
        ON OD.DEPT_CODE = DP.DEPT_CODE
 
UNION
 
SELECT
    SA.DEPT_CODE AS DEPT_CODE ,
    DP.DEPT_NAME
FROM
    TBL_SALES AS SA
    LEFT JOIN TBL_DEPT AS DP
        ON SA.DEPT_CODE = DP.DEPT_CODE
ORDER BY
    DEPT_CODE ASC;