Linq to SQL |前5個不同的排序按日期

我有一個SQL查詢，我想調用從asp.net應用程序中的LINQ到SQL。Linq to SQL |前5個不同的排序按日期

SELECT TOP 5 * 
FROM (SELECT SongId, 
       DateInserted, 
       ROW_NUMBER() 
       OVER(
        PARTITION BY SongId 
        ORDER BY DateInserted DESC) rn 
     FROM DownloadHistory) t 
WHERE t.rn = 1 
ORDER BY DateInserted DESC

我不知道它是否可能通過linq轉到sql，如果沒有的話請提供任何其他方式。

來源

2013-07-14 Manish Rawat

我想你必須將SQL分區更改爲Linq group-by。（實際上所有的分區確實是按歌曲，然後選擇最新的行爲每個組。）因此，像這樣：

IEnumerable<DownloadHistory> top5Results = DownloadHistory 
    // group by SongId 
    .GroupBy(row => row.SongId) 

    // for each group, select the newest row 
    .Select(grp => 
     grp.OrderByDescending(historyItem => historyItem.DateInserted) 
     .FirstOrDefault() 
    ) 

    // get the newest 5 from the results of the newest-1-per-song partition 
    .OrderByDescending(historyItem => historyItem.DateInserted) 
    .Take(5);

來源

2013-07-14 19:59:24 McGarnagle

它只是像魅力一樣工作。我必須說它看到快速結果真是太棒了。帽子給你McGarnagle！在這裏看到真正的工作實現：[msongs.apphb.com]（http://msongs.apphb.com） –

雖然McGarnagle回答解決了這個問題，但是當我看到這兩個執行計劃查詢，看到linq to sql與原生sql查詢相比真的太慢了，真是太神奇了。查看上面linq to sql的生成查詢：

--It took 99% of the two execution 

SELECT TOP (5) [t3].[SongId], [t3].[DateInserted] 
    FROM (
     SELECT [t0].[SongId] 
     FROM [dbo].[DownloadHistory] AS [t0] 
     GROUP BY [t0].[SongId] 
     ) AS [t1] 
    OUTER APPLY (
     SELECT TOP (1) [t2].[SongId], [t2].[DateInserted] 
     FROM [dbo].[DownloadHistory] AS [t2] 
     WHERE [t1].[SongId] = [t2].[SongId] 
     ORDER BY [t2].[DateInserted] DESC 
     ) AS [t3] 
    ORDER BY [t3].[DateInserted] DESC 


--It took 1% of the two execution 
SELECT TOP 5 t.SongId,t.DateInserted 
    FROM (SELECT SongId, 
       DateInserted, 
       ROW_NUMBER() 
       OVER(
        PARTITION BY SongId 
        ORDER BY DateInserted DESC) rn 
     FROM DownloadHistory) t 
    WHERE t.rn = 1 
    ORDER BY DateInserted DESC

來源

2013-07-14 20:25:44

Linq to SQL |前5個不同的排序按日期

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