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;