SQL按日期选择最新行

Question

我有大量的数据每10分钟左右更新一次。

有128个唯一ID需要返回，但只有最新值

当前代码

SELECT DISTINCT 
id, 
MAX(extractdate) AS [extractdate],
total,
used,
free

FROM
maintable
INNER JOIN datatable ON maintable.unkey = datatable.dataunkey

GROUP BY id, total, used, free

ORDER BY id

电流输出

id      extractdate                 total   used    free
1       2014-08-28 00:20:00.000     50      20      30
1       2014-08-28 00:30:00.000     50      30      20
1       2014-08-28 00:40:00.000     50      10      40
2       2014-08-28 00:20:00.000     50      20      30
2       2014-08-28 00:30:00.000     50      30      20
2       2014-08-28 00:40:00.000     50      25      25
etc etc

**DESIRED OUTPUT**

id      extractdate                 total   used    free
1       2014-08-28 00:40:00.000     50      10      40
2       2014-08-28 00:40:00.000     50      25      25
etc etc

Answer 1

应该可以，我刚刚在类似的秋天测试过它，只是没有加入：

SELECT id, extractdate,total,used,free
FROM maintable m INNER JOIN datatable ON m.unkey = datatable.dataunkey
where extractdate = (select max(extractdate) from manitable m1 where m1.id = m.id) 
ORDER BY id

Answer 2

尝试：

SELECT 
a.id,
a.extractdate,
a.total,
a.used,
a.free
FROM(
SELECT  
id, 
MAX(extractdate) AS [extractdate],
total,
used,
free,
ROW_NUMBER()OVER(partition by id ORDER BY MAX(extractdate) desc) AS rnk
FROM maintable
INNER JOIN datatable ON maintable.unkey = datatable.dataunkey
GROUP BY id, total, used, free )a
WHERE a.rnk = 1