寫法一：
SELECT SUBSTATION_ID,EQUIPMENT_ID MAX(GMT_EVENT) as GMT_EVENT
from ONLINE_EQUIPMENT_HEALTH GROUP BY SUBSTATION_ID,EQUIPMENT_ID寫法二：
SELECT OEH1.SUBSTATION_ID,OEH1.EQUIPMENT_ID,OEH1.GMT_EVENT
FROM ONLINE_EQUIPMENT_HEALTH OEH1
WHERER GMT_EVENT =
(SELECT MAX(GMT_EVENT) FROM ONLINE_EQUIPMENT_HEALTH OEH2
WHERE OEH1.SUBSTATION_ID = OEH2.SUBSTATION_ID AND OEH1.EQUIPMENT_ID = OEH2.EQUIPMENT_ID
)
兩種寫法有區別嗎?