C# Linq to SQL |按日期排列的前5个不同订单
我有一个SQL查询,我想在asp.net应用程序中从LINQ调用SQLC# Linq to SQL |按日期排列的前5个不同订单,c#,asp.net,linq,sql-server-2008,sql-to-linq-conversion,C#,Asp.net,Linq,Sql Server 2008,Sql To Linq Conversion,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
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
我不知道通过linq到sql是否可能,如果不可能,请提供其他方法。我认为您必须通过将sql分区更改为linq组。(实际上,分区所做的就是按歌曲分组,并为每个组选择最新的行。)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);
IEnumerable top5Results=下载历史记录
//按SongId分组
.GroupBy(row=>row.SongId)
//对于每个组,选择最新的行
.选择(grp=>
grp.OrderByDescending(historyItem=>historyItem.DateInserted)
.FirstOrDefault()
)
//从最新的每首歌曲1个分区的结果中获取最新的5个分区
.OrderByDescending(historyItem=>historyItem.DateInserted)
.采取(5);
我认为您必须将SQL分区更改为Linq group by。(实际上,分区所做的就是按歌曲分组,并为每个组选择最新的行。)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);
IEnumerable top5Results=下载历史记录
//按SongId分组
.GroupBy(row=>row.SongId)
//对于每个组,选择最新的行
.选择(grp=>
grp.OrderByDescending(historyItem=>historyItem.DateInserted)
.FirstOrDefault()
)
//从最新的每首歌曲1个分区的结果中获取最新的5个分区
.OrderByDescending(historyItem=>historyItem.DateInserted)
.采取(5);
虽然McGarnagle answer解决了这个问题,但当我看到这两个查询的执行计划时,我非常惊讶地发现linq to sql与本机sql查询相比速度太慢了。请参阅生成的查询以获取上述linq到sql:
--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
虽然McGarnagle answer解决了这个问题,但当我看到这两个查询的执行计划时,我非常惊讶地发现linq to sql与本机sql查询相比速度太慢了。请参阅生成的查询以获取上述linq到sql:
--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
它就像魅力一样。我必须说看到快速的结果真是太棒了。向你致敬McGarnagle!!!看看这里真正的工作实现:它就像魅力一样工作。我必须说看到快速的结果真是太棒了。向你致敬McGarnagle!!!请参见此处的实际工作实现: