select
    o.dept_code as DEPT_CODE
    , d.dept_name as DEPT_NAME
from tbl_orders as o
left join tbl_dept as d
    on d.dept_code=o.dept_code
union
select
    s.dept_code as DEPT_CODE
    , d.dept_name as DEPT_NAME
from tbl_sales as s
left join tbl_dept as d
    on d.dept_code=s.dept_code
order by dept_code;