with tmp as (
select
    o.dept_code DEPT_CODE
    ,d.dept_name DEPT_NAME
from
    (select distinct dept_code from tbl_orders) o
    left join tbl_dept d on d.dept_code = o.dept_code
union all
select
    s.dept_code DEPT_CODE
    ,d.dept_name DEPT_NAME
from
    (select distinct dept_code from tbl_sales) s
    left join tbl_dept d on d.dept_code = s.dept_code

)
select
    distinct *
from
    tmp
order by
    dept_code asc
;