select "港コード", "港名", "入国者数", "出国者数", "入国者数" - "出国者数" as "差分"
from (
    select
        port_code as "港コード",
        port_name as "港名",
        (
            select sum(amt)
            from immigration as i 
            where i.port_code = p.port_code and group_code = 110 and kind_code = 110
        ) as "入国者数",
        (
            select sum(amt)
            from immigration as i 
            where i.port_code = p.port_code and group_code = 110 and kind_code = 120
        ) as "出国者数"
    from port as p
    )
where "差分" > 0
order by "差分" desc, "港コード" desc;