[英]Select distinct rows with max value of another column
我正在查询文档管理数据库,其中文档可以有多个版本。 因此,由于版本控制,我下面的查询多次返回相同的文档。
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'
结果:
DOCNUM | DOCNAME | FOLDERNAME |
-----------+----------+---------------+
123 Article Jonathan
123 Article part1 Jonathan
256 Meeting Notes Jonathan
5697 Memo Jonathan
理想情况下 docnum 123 应该只返回一次。
我加入了保存每个文档版本的表,并选择了最新版本(MAX(h.version)
因此我的查询应该每行返回唯一的文档编号。
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)
但是这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。 请参阅下面的文档 123。
DOCNUM | Version | DOCNAME | FOLDERNAME |
-----------+--------------+-------------+------------+
123 9 Article Jonathan
123 9 Article part1 Jonathan
256 1 Meeting Notes Jonathan
5697 21 Memo Jonathan
我需要在我的报告中显示 docname 列。 请问这个有办法吗?
是的,您可以选择前 1 个文档名。 将您的字段选择列表替换为:
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
....(其余代码)
在上面的输出中,doc 123 version 9 应该显示为 Article。
使用窗口函数,您要做的事情要容易得多。 这是一种方法:
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;
表达式max(h.version) over (partition by d.docnum)
是一种称为窗口函数(或某些数据库中的分析函数max(h.version) over (partition by d.docnum)
的构造。 所做的是为h.version
的每个值返回h.version
的d.docnum
(基于partition by
子句)。 这是 ANSI 标准 SQL,大多数数据库都支持此功能。
您的版本可能无法正常工作,因为计算最大值的子查询未使用与外部查询相同的过滤条件。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.