SELECT
    ORDERS.DEPT_CODE,
    DEPT.DEPT_NAME
FROM
    TBL_ORDERS AS ORDERS
LEFT OUTER JOIN
    TBL_DEPT AS DEPT
ON  DEPT.DEPT_CODE = ORDERS.DEPT_CODE
UNION
SELECT
    SALES.DEPT_CODE,
    DEPT.DEPT_NAME
FROM
    TBL_SALES AS SALES
LEFT OUTER JOIN
    TBL_DEPT AS DEPT
ON  DEPT.DEPT_CODE = SALES.DEPT_CODE