mysql：
SELECT * from
(SELECT H_TEMPERATURE,TH_TIME FROM wenshidu
WHERE TH_TIME <= STR_TO_DATE('2016-03-16 10:04:52','%Y-%m-%d %H:%i:%s')
order by TH_TIME desc) as total limit 0,8;
oracle：
SELECT * from
(SELECT H_HUMIDITY FROM HOME_MONITOR
WHERE H_TIME <= to_date( ' 2016-03-01 10:04:52 ' ,'yyyy-MM-dd HH24:mi:ss')
order by H_TIME desc ) where rownum<=8;