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