SELECT
    TCS.DEPT_CODE,
    TD.DEPT_NAME
FROM
    (SELECT DISTINCT DEPT_CODE FROM TBL_ORDERS
    UNION SELECT DISTINCT DEPT_CODE FROM TBL_SALES) AS TCS
    LEFT OUTER JOIN TBL_DEPT AS TD ON TCS.DEPT_CODE = TD.DEPT_CODE
ORDER BY
    TCS.DEPT_CODE ASC;