with t as (
    select PORT_CODE as "港コード",
            PORT_NAME as "港名",
            SUM(case KIND_CODE when '110' then AMT else 0 end) as "入国者数",
            SUM(case KIND_CODE when '120' then AMT else 0 end) as "出国者数"
    from IMMIGRATION
        inner join
            PORT
        using(PORT_CODE)
    where GROUP_CODE = '120'
    group by PORT_CODE , PORT_NAME
    )

select * , "入国者数"-"出国者数" as "差分"
from t
where "差分" > 0
order by "差分" desc, "港コード" desc
;