select *,  `入国者数` - `出国者数` as `差分`
from (
    select 
        p.PORT_CODE as 港コード,
        max(p.PORT_NAME) as 港名,
        sum(CASE WHEN im.KIND_CODE = '110' and g.GROUP_NAME = '外国人' THEN im.AMT ELSE 0 END) as '入国者数',
        sum(CASE WHEN im.KIND_CODE = '120' and g.GROUP_NAME = '外国人' THEN im.AMT ELSE 0 END) as '出国者数'
    from IMMIGRATION im
        join PORT p on p.PORT_CODE = im.PORT_CODE
        join GRP g on g.GROUP_CODE = im.GROUP_CODE 
    group by p.PORT_CODE
)
where `入国者数` - `出国者数` > 0
order by `差分` desc, `港コード` desc