如何選擇列的最大值

Question

我的問題似乎很簡單，但我無法解決。

這就是我所擁有的：

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

這就是我想要的：

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

我已經試過了：

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;

這讓我有點想要，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

這給了我正確的版本，但是數據集的大小並沒有減少，這就是我想要的。

任何幫助都非常可觀！

Answer 1

您在尋找GROUP BY嗎？

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

Answer 2

您需要修復日期算法！ 在查詢中始終使用ISO標准YYYY-MM-DD（或YYYYMMDD）格式。 不要使用字符串進行日期比較。

那么，您的問題的答案是一個簡單的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;

假設您在referencetid上有一個索引，我建議您只顯式列出這些值：

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;

這使查詢更易於使用索引進行優化。

Answer 3

今天似乎有很多問題要問過去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 （或其他函數）作為過濾器的一部分將阻止查詢使用該列上的任何索引（您將需要基於函數的索引）。

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

您可以使用此代碼

Answer 5

正如許多人已經回答的那樣，出於某種考慮，您在尋找分組依據，請在此處查看此示例或其他示例以學習如何使用它，這是一個很好的服務來嘗試查詢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;