Question

我有一个SQL查询，我想在asp.net应用程序中从LINQ调用SQL。

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

我不知道是否有可能通过linq到sql，如果没有那么请提供任何其他方式。

Answer 1

我认为您必须将SQL分区更改为Linq group-by。（实际上所有分区都是逐个分组，并为每个组选择最新的行。）所以像这样：

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);

Answer 2

尽管McGarnagle的回答解决了这个问题，但是当我看到两个查询的执行计划时，看到linq to sql与本机sql查询相比真的太慢了，真是太神奇了。请参阅上面的linq到sql生成的查询：

--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

Linq to SQL |排名前5位的不同订单

2 个答案: