Question

给定这样的表格，每个监视器的最新校准信息是什么？换句话说，我想找到每个监视器的最大日期值。特定于Oracle的功能适用于我的应用程序。

monitor_id     calibration_date  value
----------     ----------------  -----
1              2011/10/22        15
1              2012/01/01        16
1              2012/01/20        17
2              2011/10/22        18
2              2012/01/02        19

此示例的结果如下所示：

1  2012/01/20 17
2  2012/01/02 19

Answer 1

我倾向于使用分析函数

SELECT monitor_id,
       host_name,
       calibration_date,
       value
  FROM (SELECT b.monitor_id,
               b.host_name,
               a.calibration_date,
               a.value,
               rank() over (partition by b.monitor_id order by a.calibration_date desc) rnk
          FROM table_name a,
               table_name2 b
         WHERE a.some_key = b.some_key)
 WHERE rnk = 1

您也可以使用相关子查询，但效率会降低

SELECT monitor_id,
       calibration_date,
       value
  FROM table_name a
 WHERE a.calibration_date = (SELECT MAX(b.calibration_date)
                               FROM table_name b
                              WHERE a.monitor_id = b.monitor_id)

Answer 2

我个人的偏好是：

SELECT DISTINCT
       monitor_id
      ,MAX(calibration_date)
       OVER (PARTITION BY monitor_id)
       AS latest_calibration_date
      ,FIRST_VALUE(value)
       OVER (PARTITION BY monitor_id
             ORDER BY calibration_date DESC)
       AS latest_value
FROM mytable;

变体就是使用FIRST_VALUE的{{1}}语法。无论哪种方式都有效。

Answer 3

窗口函数解决方案应该最有效，并且只能进行一次表或索引扫描。我在这里发帖的那个我认为通过直观和易于理解赢得了一些观点。我在SQL服务器上进行了测试，它在窗口函数中执行了第二次，导致了两次索引扫描。

SELECT T1.monitor_id, T1.calibration_date, T1.value
FROM someTable AS T1
WHERE NOT EXISTS 
(
    SELECT * 
    FROM someTable AS T2
    WHERE T2.monitor_id = T1.monitor_id AND T2.value > T1.value
)
 GROUP BY T1.monitor_id, T1.calibration_date, T1.value

仅仅是为了它，这是另一个沿着相同的路线，但性能较差（63％成本对比37％）比另一个（再次在sql server中）。这个在执行计划中使用Left Outer Join，其中第一个使用Anti-Semi Merge Join：

SELECT T1.monitor_id, T1.calibration_date, T1.value
FROM someTable AS T1
LEFT JOIN someTable AS T2 ON T2.monitor_id = T1.monitor_id AND T2.value > T1.value
WHERE T2.monitor_id IS NULL
GROUP BY T1.monitor_id, T1.calibration_date, T1.value

Answer 4

select monitor_id, calibration_date, value 
from table
where calibration_date in(
  select max(calibration_date) as calibration_date 
  from table
  group by monitor_id
  )

Oracle：获得团队的最大价值？

4 个答案: