select
a.PORT_CODE AS 港コード,
c.PORT_NAME AS 港名,

sum(a.AMT) as 入国者数 , 
sum(b.AMT) as 出国者数,
sum(a.AMT) - sum(b.AMT) as 差分
from 
IMMIGRATION a 
left join  IMMIGRATION b on a.port_code=b.port_code
and a.kind_code='110' and b.kind_code='120'
left join PORT c on a.port_code=c.port_code
left join grp g on a.group_code=g.group_code
where a.group_code='120' and b.group_code='120'

group by a.port_code
having sum(a.AMT) - sum(b.AMT) >0