SELECT S1.DEPT_CODE AS DEPT_CODE
,D1.DEPT_NAME
FROM TBL_SALES AS S1
LEFT JOIN TBL_DEPT AS D1
ON S1.DEPT_CODE = D1.DEPT_CODE

UNION 

SELECT O1.DEPT_CODE AS DEPT_CODE
,D1.DEPT_NAME
FROM TBL_ORDERS AS O1
LEFT JOIN TBL_DEPT AS D1
ON O1.DEPT_CODE = D1.DEPT_CODE

ORDER BY
    DEPT_CODE ASC;