with
pre as (
select
  imm.PORT_CODE
, sum(case when imm.KIND_CODE = '110' then imm.AMT else 0 end) as "入国者数"
, sum(case when imm.KIND_CODE = '120' then imm.AMT else 0 end) as "出国者数"
, sum(case when imm.KIND_CODE = '110' then imm.AMT else 0 end
    - case when imm.KIND_CODE = '120' then imm.AMT else 0 end) as "差分"
from
  IMMIGRATION as imm
where
  imm.GROUP_CODE = '120'
group by
  imm.PORT_CODE
)
select 
  por.PORT_CODE as "港コード"
, por.PORT_NAME as "港名"
, case when pre."入国者数" is null then 0 else pre."入国者数" end as "入国者数"
, case when pre."出国者数" is null then 0 else pre."出国者数" end as "出国者数"
, case when pre."差分"     is null then 0 else pre."差分"     end as "差分"
from
 PORT por
left outer join pre
on por.PORT_CODE = pre.PORT_CODE
order by
  case when pre."差分"     is null then 0 else pre."差分"     end desc
, por.PORT_CODE desc
;