如何选择列的最大值

Question

我的问题似乎很简单，但我无法解决。

这就是我所拥有的：

TIME           MAXVERSION
13-12-31          1
13-12-31          2
14-12-31          1
14-12-31          2
14-12-31          3
15-12-31          1
15-12-31          2
15-12-31          3
16-12-31          1

这就是我想要的：

TIME           MAXVERSION
13-12-31          2
14-12-31          3
15-12-31          3
16-12-31          1

我已经试过了：

select referencetid as time max(version) over (partition by referencetid order by version desc) as maxversion
from D101200.PSD_BEFOLKNING_REFERENCE
where to_char(referencetid, 'mm-dd') = '12-31'
and to_char(referencetid, 'yy-mm-dd') between ('07-12-31') and ('16-12-31')
order by referencetid;

这让我有点想要，但不完全是：

TIME           MAXVERSION
13-12-31          2
13-12-31          2
14-12-31          3
14-12-31          3
14-12-31          3
15-12-31          3
15-12-31          3
15-12-31          3
16-12-31          1

这给了我正确的版本，但是数据集的大小并没有减少，这就是我想要的。

任何帮助都非常可观！

Answer 1

您在寻找GROUP BY吗？

     select Time,
            Max(MaxVersion) as MaxVersion  
       from MyTable
   group by Time
   order by Time

Answer 2

您需要修复日期算法！ 在查询中始终使用ISO标准YYYY-MM-DD（或YYYYMMDD）格式。 不要使用字符串进行日期比较。

那么，您的问题的答案是一个简单的group by ：

select referencetid as time, max(version)
from D101200.PSD_BEFOLKNING_REFERENCE
where to_char(referencetid, 'mm-dd') = '12-31' and
      referencetid between date '2007-12-31' and date '2016-12-31'
group by referencetid
order by referencetid;

假设您在referencetid上有一个索引，我建议您只显式列出这些值：

select referencetid as time, max(version)
from D101200.PSD_BEFOLKNING_REFERENCE
where referencetid in (date '2007-12-31', date '2008-12-31', date '2009-12-31',
                       date '2010-12-31', date '2011-12-31', date '2012-12-31',
                       date '2013-12-31', date '2014-12-31', date '2015-12-31',
                       date '2016-12-31')
group by referencetid
order by referencetid;

这使查询更易于使用索引进行优化。

Answer 3

今天似乎有很多问题要问过去N年的12月31日左右

SELECT referencetid as time,
       max(version) as maxversion
FROM   D101200.PSD_BEFOLKNING_REFERENCE
WHERE  referencetid IN (
         DATE '2016-12-31',
         DATE '2015-12-31',
         DATE '2014-12-31',
         DATE '2013-12-31',
         DATE '2012-12-31',
         DATE '2011-12-31',
         DATE '2010-12-31',
         DATE '2009-12-31',
         DATE '2008-12-31',
         DATE '2007-12-31'
       )
GROUP BY referencetid
ORDER BY referencetid;

在日期列上使用TO_CHAR （或其他函数）作为过滤器的一部分将阻止查询使用该列上的任何索引（您将需要基于函数的索引）。

Answer 4

select 
referencetid as time, 
max(version)as maxversion
from D101200.PSD_BEFOLKNING_REFERENCE
where to_char(referencetid, 'mm-dd') = '12-31'
and to_char(referencetid, 'yy-mm-dd') between ('07-12-31') and ('16-12-31')
group by referencetid 
order by referencetid

您可以使用此代码

Answer 5

正如许多人已经回答的那样，出于某种考虑，您在寻找分组依据，请在此处查看此示例或其他示例以学习如何使用它，这是一个很好的服务来尝试查询sqlfiddle

select 
    referencetid as Time,
    max(version) as maxversion

from D101200.PSD_BEFOLKNING_REFERENCE

where to_char(referencetid, 'mm-dd') = '12-31'
   and to_char(referencetid, 'yy-mm-dd') between ('07-12-31') and ('16-12-
 31')
group by referencetid 
order by referencetid;