SELECT 
    tbl.DEPT_CODE,
    d.DEPT_NAME
FROM (
    SELECT 
        DISTINCT o.DEPT_CODE
    FROM TBL_ORDERS AS o
    FULL JOIN TBL_SALES AS s ON s.DEPT_CODE = o.DEPT_CODE
    WHERE o.DEPT_CODE IS NOT NULL
    ) AS tbl
LEFT JOIN TBL_DEPT AS d ON tbl.DEPT_CODE = d.DEPT_CODE
ORDER BY tbl.DEPT_CODE;