SELECT 
    tbl.DEPT_CODE,
    d.DEPT_NAME
FROM (
    SELECT DISTINCT o.DEPT_CODE FROM TBL_ORDERS AS o
    UNION
    SELECT DISTINCT s.DEPT_CODE FROM TBL_SALES AS s
    ) AS tbl
LEFT OUTER JOIN TBL_DEPT AS d ON d.DEPT_CODE = tbl.DEPT_CODE
ORDER BY tbl.DEPT_CODE;