SELECT
OD.DEPT_CODE AS DEPT_CODE,
DP.DEPT_NAME

FROM TBL_ORDERS AS OD
LEFT OUTER JOIN TBL_DEPT AS DP
 ON DP.DEPT_CODE = OD.DEPT_CODE
 
UNION

SELECT 
SA.DEPT_CODE AS DEPT_CODE
,DP.DEPT_NAME

FROM TBL_SALES AS SA
LEFT OUTER JOIN TBL_DEPT AS DP
 ON DP.DEPT_CODE = SA.DEPT_CODE
ORDER BY 
DEPT_CODE ASC;