Question

在阅读了Stackoverflow很长一段时间之后，今天轮到我问了一个问题！

表格结构
它与SQL Server 2005数据库的查询有关。我有以下表结构：

部件

ID

下载

ID
componentid（FK to components＃id）
downloaddate

进口

ID
downloadid（FK to downloads＃id）
状态

我想得到什么
我正在尝试获取每个组件的最新下载状态。

我到目前为止
我已经开始编写此查询，但我的联接工作效果不佳：

SELECT c.id  
FROM components as c  
JOIN (    
  SELECT componentid, MAX(downloaddate) as downloaddate, id  
  FROM downloads  
  GROUP BY componentid, id   
) d ON d.componentid = c.id  
-- JOIN on imports here...

我的问题是用于连接的查询不正确。因为我需要为连接选择id，所以我在group by子句中添加了它，现在我为每个组件获取多条记录，而不是最新的组件。

如果我自己在连接中执行查询，这就是我得到的。

componentid downloaddate            id

8882    2011-01-15 06:00:09.773 595862
8882    2011-01-16 06:00:09.847 598422
8882    2011-01-17 06:00:09.940 600971
8951    2010-11-15 03:00:22.550 450221
8951    2010-11-16 03:00:21.730 452431
8951    2010-11-17 03:00:21.920 454668
8961    2011-01-17 02:00:33.077 600532
8961    2011-01-17 02:00:59.070 600536
8983    2008-07-17 11:59:53.780 122
9005    2009-09-01 04:00:38.320 48918
9005    2009-09-02 04:00:36.260 49280

相反，我需要这样的事情：

componentid downloaddate            id

8882    2011-01-17 06:00:09.940 600971
8951    2010-11-17 03:00:21.920 454668
8961    2011-01-17 02:00:59.070 600536
8983    2008-07-17 11:59:53.780 122
9005    2009-09-02 04:00:36.260 49280

有谁知道如何编写我的联接以达到预期的结果？

提前多多感谢

Answer 1

您无法关联JOIN运算符。请改用APPLY：

SELECT c.id , d.downloaddate
FROM components as c  
APPLY (    
  SELECT componentid, MAX(downloaddate) as downloaddate  
  FROM downloads  
  WHERE componentid = c.id
) as d;

实际上你也可以使用JOIN（因为内部查询实际上没有关联），但是不按ID分组，只是按组件分组：

SELECT c.id , d.downloaddate
FROM components as c  
JOIN (    
  SELECT componentid, MAX(downloaddate) as downloaddate  
  FROM downloads  
  GROUP BY componentid 
) as d ON c.id = d.componentid;

更新：

SELECT *
FROM components as c  
APPLY (    
  SELECT TOP(1) componentid, downloaddate, id as downloadid
  FROM downloads  
  WHERE componentid = c.id
  ORDER BY downloaddate DESC
) as d
JOIN imports i on i.id = d.downloadid;

Answer 2

您可以使用ROW_NUMBER函数解决此问题：

SELECT componentid, ROW_NUMBER() OVER (PARTITION BY componentid ORDER BY downloaddate DESC) as rowno
FROM downloads  
WHERE rowno = 1

SQL服务器 - 获取最新的连接记录

2 个答案: