-- わかった

select
im_in.PORT_CODE as 港コード,
po.PORT_NAME as 港名,
im_in.AMT as 入国者数,
im_out.AMT as 出国者数,
(im_in.AMT - im_out.AMT) as 差分

from
IMMIGRATION as im_in
inner join GRP as gr
on im_in.GROUP_CODE = gr.GROUP_CODE
inner join PORT as po
on im_in.PORT_CODE = po.PORT_CODE
inner join IMMIGRATION as im_out
on im_in.PORT_CODE = im_out.PORT_CODE
-- ↓が抜けてたのか。ポート番号が同じでもグループが違う可能性、あるもんね・・・
and im_in.GROUP_CODE = im_out.GROUP_CODE
and im_out.KIND_CODE = "120"

where
im_in.KIND_CODE = "110"
and im_in.GROUP_CODE = "120"
and 差分 > 0

order by
差分 desc, 港コード desc;