SELECT b.DEPT_CODE,  a.DEPT_NAME
FROM TBL_ORDERS AS b

FULL OUTER JOIN TBL_DEPT AS a
ON b.DEPT_CODE = a.DEPT_CODE

UNION

SELECT c.DEPT_CODE,  a.DEPT_NAME
FROM TBL_SALES AS c 
FULL OUTER JOIN TBL_DEPT AS a
ON c.DEPT_CODE = a.DEPT_CODE