SELECT 
    DISTINCT(ORD.DEPT_CODE) AS DEPT_CODE
    ,DEP.DEPT_NAME
FROM TBL_ORDERS AS ORD 
LEFT OUTER JOIN TBL_DEPT AS DEP
    ON ORD.DEPT_CODE = DEP.DEPT_CODE
UNION

SELECT 
    DISTINCT(SALES.DEPT_CODE) AS DEPT_CODE
    ,DEP.DEPT_NAME
FROM TBL_SALES AS SALES 
LEFT OUTER JOIN TBL_DEPT AS DEP
    ON SALES.DEPT_CODE = DEP.DEPT_CODE
ORDER BY DEPT_CODE;