SELECT DISTINCT
  orders.DEPT_CODE,
  dept.DEPT_NAME
FROM
  TBL_ORDERS AS orders
  LEFT OUTER JOIN TBL_SALES AS sales ON orders.DEPT_CODE = sales.DEPT_CODE
  LEFT OUTER JOIN TBL_DEPT AS dept ON orders.DEPT_CODE = dept.DEPT_CODE
ORDER BY
  orders.DEPT_CODE asc;