[英]SQL server - Get newest record of join
在閱讀了Stackoverflow一段時間之后,今天輪到我來提一個問題!
表結構
它與對SQL Server 2005數據庫的查詢有關。 我具有以下表結構:
組件
資料下載
進口
我想得到什么
我正在嘗試獲取每個組件的最新下載狀態。
到目前為止我在哪里
我已經開始編寫此查詢,但是我的聯接工作得不太好:
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...
我的問題是用於聯接的查詢不正確。 因為我需要選擇連接的ID,所以將其添加到group by子句中,現在我獲得了每個componentid的多個記錄,而不是最新的記錄。
這是我自己在聯接中執行查詢時得到的結果。
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,我需要這樣的東西:
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
有誰知道我應該怎么寫我的聯接來達到預期的結果?
在此先多謝
您無法關聯JOIN運算符。 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;
實際上,您也可以使用JOIN(因為內部查詢實際上沒有關聯),但不要按id分組,而按componentid分組:
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;
更新時間 :
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;
您可能可以使用ROW_NUMBER函數解決此問題:
SELECT componentid, ROW_NUMBER() OVER (PARTITION BY componentid ORDER BY downloaddate DESC) as rowno
FROM downloads
WHERE rowno = 1
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