[英]Select distinct rows with max value of another column
我正在查詢文檔管理數據庫,其中文檔可以有多個版本。 因此,由於版本控制,我下面的查詢多次返回相同的文檔。
SELECT distinct d.DOCNUM, d.DOCNAME,P.FolderName
FROM DOCUMENT as d with (nolock)
inner join Project_T as pt on d.docnum=pt.item_id
inner join Projects p on pt.prj_id=p.prj_id
where d.type = 'Personal' and d.owner like '%67360'
結果:
DOCNUM | DOCNAME | FOLDERNAME |
-----------+----------+---------------+
123 Article Jonathan
123 Article part1 Jonathan
256 Meeting Notes Jonathan
5697 Memo Jonathan
理想情況下 docnum 123 應該只返回一次。
我加入了保存每個文檔版本的表,並選擇了最新版本(MAX(h.version)
因此我的查詢應該每行返回唯一的文檔編號。
SELECT distinct d.DOCNUM, h.version, d.DOCNAME,P.FolderName
FROM DOCUMENT as d with (nolock)
inner join DOCHISTORY as h on h.DOCNUM = d.docnum
inner join Project_T as pt on d.docnum=pt.item_id
inner join Projects p on pt.prj_id=p.prj_id
where d.type = 'Personal' and d.owner like '%67360'
and d.SECURITY = 'P'
AND h.VERSION = (SELECT MAX(x.version) FROM DOCHISTORY as X
where x.docnum = d.DOCNUM)
但是這次我得到了最新版本,其中 docname 略有不同,它多次返回相同的 docnum。 請參閱下面的文檔 123。
DOCNUM | Version | DOCNAME | FOLDERNAME |
-----------+--------------+-------------+------------+
123 9 Article Jonathan
123 9 Article part1 Jonathan
256 1 Meeting Notes Jonathan
5697 21 Memo Jonathan
我需要在我的報告中顯示 docname 列。 請問這個有辦法嗎?
是的,您可以選擇前 1 個文檔名。 將您的字段選擇列表替換為:
SELECT distinct d.DOCNUM
,h.version
,DOCNAME = (SELECT TOP 1 d2.DOCNAME FROM DOCUMENT d2, DOCHISTORY h2
WHERE d2.DOCNUM = d.DOCNUM and h2.DOCNUM = d.docnum)
,P.FolderName
....(其余代碼)
在上面的輸出中,doc 123 version 9 應該顯示為 Article。
使用窗口函數,您要做的事情要容易得多。 這是一種方法:
select DOCNUM, version, DOCNAME, FolderName
from (SELECT d.DOCNUM, h.version, d.DOCNAME, P.FolderName,
max(h.version) over (partition by d.docnum) as maxversion
FROM DOCUMENT d with (nolock)
inner join DOCHISTORY as h on h.DOCNUM = d.docnum
inner join Project_T as pt on d.docnum=pt.item_id
inner join Projects p on pt.prj_id=p.prj_id
where d.type = 'Personal' and d.owner like '%67360' and d.SECURITY = 'P'
) d
where version = maxversion;
表達式max(h.version) over (partition by d.docnum)
是一種稱為窗口函數(或某些數據庫中的分析函數max(h.version) over (partition by d.docnum)
的構造。 所做的是為h.version
的每個值返回h.version
的d.docnum
(基於partition by
子句)。 這是 ANSI 標准 SQL,大多數數據庫都支持此功能。
您的版本可能無法正常工作,因為計算最大值的子查詢未使用與外部查詢相同的過濾條件。
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