Question

我有一个LinqToEntities查询，在创建SQL时会生成一个子查询。每次运行查询时，这会导致结果集返回0-3结果。子查询本身会产生一个随机结果（应该如此）。这是怎么回事？

LINQ查询：

from jpj in JobProviderJobs
where jpj.JobID == 4725 
&& jpj.JobProviderID == (from jp2 in JobProviderJobs 
                        where jp2.JobID == 4725 
                        orderby Guid.NewGuid() 
                        select jp2.JobProviderID).FirstOrDefault()
select new 
{
    JobProviderID = jpj.JobProviderID
}

制作此SQL：

SELECT 
[Filter2].[JobID] AS [JobID], 
[Filter2].[JobProviderID1] AS [JobProviderID]
FROM   (SELECT [Extent1].[JobID] AS [JobID], [Extent1].[JobProviderID] AS [JobProviderID1], [Limit1].[JobProviderID] AS [JobProviderID2]
    FROM  [dbo].[JobProviderJob] AS [Extent1]
    LEFT OUTER JOIN  (SELECT TOP (1) [Project1].[JobProviderID] AS [JobProviderID]
        FROM ( SELECT 
            NEWID() AS [C1], 
            [Extent2].[JobProviderID] AS [JobProviderID]
            FROM [dbo].[JobProviderJob] AS [Extent2]
            WHERE 4725 = [Extent2].[JobID]
        )  AS [Project1]
        ORDER BY [Project1].[C1] ASC ) AS [Limit1] ON 1 = 1
    WHERE 4725 = [Extent1].[JobID] ) AS [Filter2]
LEFT OUTER JOIN  (SELECT TOP (1) [Project2].[JobProviderID] AS [JobProviderID]
    FROM ( SELECT 
        NEWID() AS [C1], 
        [Extent3].[JobProviderID] AS [JobProviderID]
        FROM [dbo].[JobProviderJob] AS [Extent3]
        WHERE 4725 = [Extent3].[JobID]
    )  AS [Project2]
    ORDER BY [Project2].[C1] ASC ) AS [Limit2] ON 1 = 1
WHERE [Filter2].[JobProviderID1] = (CASE WHEN ([Filter2].[JobProviderID2] IS NULL) THEN 0 ELSE [Limit2].[JobProviderID] END)

修改

所以将子查询更改为可行，但我不知道为什么

(from jp2 in JobProviderJobs 
    where jp2.JobID == 4725 
    orderby Guid.NewGuid() 
    select jp2).FirstOrDefault().JobProviderID

Answer 1

由于FirstOrDefault()的预期行为，它正在执行此操作。在一组空的JobProviderJobs上调用FirstOrDefault()会产生null值，但在一组空int s上调用它会产生0。认识到这一点，LINQ to Entities尝试在查询结束时调用case语句，以确保如果没有匹配的JobProviderJobs，则select的结果将为0而不是null。

在大多数情况下，重新创建内部投影不会产生任何问题，但使用NewGuid()显然会抛弃此逻辑。

你找到了一个解决方案。另一个是将内在表达的结果转换为：

from jpj in JobProviderJobs
where jpj.JobID == 4725 
&& jpj.JobProviderID == (from jp2 in JobProviderJobs 
                        where jp2.JobID == 4725 
                        orderby Guid.NewGuid() 
                        select (int?) jp2.JobProviderID).FirstOrDefault()
select new 
{
    JobProviderID = jpj.JobProviderID
}

更正确的实现将重用初始SQL投影，而不是创建两个等效投影。解析器很可能不够复杂，无法正确处理，并且开发人员从未发现需要修复它，因为SQL Server应该能够优化相同的投影，只要它们是确定性的。

如果还不存在，您应该记录一个关于此的错误报告。

LinqToEntities产生不正确的SQL（Doubled子查询）

1 个答案: