select 
 p.PORT_CODE AS '港コード',
 p.PORT_NAME AS '港名',
 i1.AMT AS '入国者数',
 i2.AMT AS '出国者数',
 i1.AMT - i2.AMT AS '差分'
from PORT p 
LEFT JOIN IMMIGRATION i1 ON p.PORT_CODE = i1.PORT_CODE and i1.KIND_CODE='110'
LEFT JOIN GRP g1 ON i1.GROUP_CODE = g1.GROUP_CODE and g1.GROUP_NAME='外国人'
LEFT JOIN IMMIGRATION i2 ON p.PORT_CODE = i2.PORT_CODE and i2.KIND_CODE='120'
LEFT JOIN GRP g2 ON i2.GROUP_CODE = g2.GROUP_CODE and g2.GROUP_NAME='外国人'
WHERE i1.AMT > i2.AMT;