WITH
  ALL_DEPT AS (
    SELECT DISTINCT
      orders.DEPT_CODE
    FROM
      TBL_ORDERS AS orders
    UNION
    SELECT DISTINCT
      sales.DEPT_CODE
    FROM
      TBL_SALES AS sales
  )
SELECT
  ALL_DEPT.DEPT_CODE,
  DEPT_NAME
FROM
  ALL_DEPT
  LEFT OUTER JOIN TBL_DEPT ON ALL_DEPT.DEPT_CODE = TBL_DEPT.DEPT_CODE
ORDER BY
  ALL_DEPT.DEPT_CODE asc;