select a.PORT_CODE as 港コード
      ,b.PORT_NAME as 港名
      ,ifnull(a.amt_in,0) as 入国者数
      ,ifnull(a.amt_out,0) as 出国者数
      ,ifnull(a.amt_in,0) - ifnull(a.amt_out,0) AS 差分
from (
select a.PORT_CODE
      ,sum(case when KIND_CODE = '110' THEN a.AMT ELSE 0 END) as amt_in
      ,sum(case when KIND_CODE = '120' THEN a.AMT ELSE 0 END) as amt_out
from IMMIGRATION a
left join GRP b
on  a.GROUP_CODE = b.GROUP_CODE
where b.GROUP_NAME = '外国人'
group by a.PORT_CODE
)a
left join PORT b
on a.PORT_CODE = b.PORT_CODE
where ifnull(a.amt_in,0) > ifnull(a.amt_out,0)
order by ifnull(a.amt_in,0) - ifnull(a.amt_out,0) desc,a.PORT_CODE desc