C#, linq to sql, 为嘛single生成的sql 是top (2)而不是top (1)

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SELECT TOP (2)
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name],
[Extent1].[Url] AS [Url],
[Extent1].[ParentId] AS [ParentId]
FROM [dbo].[MenuItem] AS [Extent1]
WHERE [Extent1].[ParentId] IS NULL

Extent1

ParentId

top

10 replies • 1970-01-01 08:00:00 +08:00

avatasia

Apr 20, 2012

好吧，single 如果结果多于2个就抛出异常

thereisnoif

Apr 24, 2012

生成的sql应该是：
SELECT [t0].[id] as [id]
FROM [dbo].[table] as [t0]
WHERE [t0].[id] IS NULl
这样的，没有TOP语句；

Single扩展方法Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.

http://msdn.microsoft.com/en-us/library/bb535118.aspx

avatasia

Apr 24, 2012

@thereisnoif 你错了

thereisnoif

Apr 24, 2012

@avatasia 我用linqpad实验的结果显示，Single生成的sql是没有Top的；First会产生Top语句；不过http://stackoverflow.com/questions/8829314/why-does-single-linq-extension-method-create-a-query-select-top2 确实是如你所说的情况。
也许是Linqpad显示的sql有问题？
明天去看看Single源码去

Ricepig

Apr 25, 2012

是要检查是不是多于1个吧。这个时候返回top 2不是很合理？

avatasia

Apr 25, 2012

@thereisnoif 你要用sql server profiler去看

avatasia

Apr 25, 2012

@Ricepig single 是取唯一的一个，first是取多个中的第一个，所以single要做多于1的判断

thereisnoif

Apr 25, 2012

@avatasia 很奇怪啊，我确实用sql profiler看了，是没有top的。。。
难道跟Linq版本有关，我是4.0；
C#查询是：
var ret = App_Case_Alls.Single(c => c.Case_type == 1);
Console.Write(ret);

sql profilerie截到的sql是：

exec sp_executesql N'SELECT [t0].[case_id] AS [Case_id], [t0].[case_type] AS [Case_type], [t0].[req_no] AS [Req_no]
FROM [app_Case_All] AS [t0]
WHERE [t0].[case_type] = @p0',N'@p0 int',@p0=1

Ricepig

Apr 25, 2012

@avatasia 我说的就是这个意思。。。。

thereisnoif

Apr 25, 2012

@avatasia 终于弄清楚了，用Entity Framework的Linq查询Single，确实生成的sql是TOP(2); 但用Linq-to-Sql的话，Single是不会有产生TOP语句的；我的实验都是直接用Linq-to-sql的，所以才产生困扰。see http://stackoverflow.com/questions/10309100/does-linq2sql-generated-sql-contains-top-or-not-when-single-is-used