with base as (
    SELECT
        port_code
    ,   kind_code
    ,   SUM(amt) as num
    FROM
        immigration as i
        inner join
            grp as g
            on i.group_code = g.group_code
    WHERE
        g.group_name = '外国人'
    GROUP BY
        kind_code
    ,   port_code
)
select
    b1.port_code AS 港コード
,   p.port_name as 港名
,   b1.num as 入国者数
,   b2.num as 出国者数
,   b1.num - b2.num as 差分
from
    base as b1
    inner join
        base as b2
        on b1.port_code = b2.port_code
    inner join
        port as p
        on b1.port_code = p.port_code
where
    b1.kind_code = '110'
    and
    b2.kind_code = '120'
    and
    b1.num > b2.num
ORDER BY
    差分 desc
,   b1.port_code DESC