Join LINQ到实体在实例而不是id上连接会生成SQL

有人能解释一下，为什么按实体而不是id联接会生成一些非常难看的sql，而实际上它在概念上做的事情与您认为的是一样的吗？e、 g

凭身份证

产生

FROM  [COMPANY] AS [Extent1]
    INNER JOIN [ADDRESS] AS [Extent2] ON [Extent1].[CONTACT_ADDRESS_ID] = [Extent2].[CONTACT_ADDRESS_ID]

举例来说

from companyDirector in CompanyDirectors
join contactAddress in ContactAddresses
  on companyDirector.ContactAddress equals contactAddress
select new {companyDirector, contactAddress}

产生

FROM  [COMPANY] AS [Extent1]
INNER JOIN [ADDRESS] AS [Extent2] ON  EXISTS (SELECT 
    1 AS [C1]
    FROM    ( SELECT 1 AS X ) AS [SingleRowTable1]
    LEFT OUTER JOIN  (SELECT 
        [Extent3].[CONTACT_ADDRESS_ID] AS [CONTACT_ADDRESS_ID]
        FROM [ADDRESS] AS [Extent3]
        WHERE [Extent1].[CONTACT_ADDRESS_ID] = [Extent3].[CONTACT_ADDRESS_ID] ) AS [Project1] ON 1 = 1
    LEFT OUTER JOIN  (SELECT 
        [Extent4].[CONTACT_ADDRESS_ID] AS [CONTACT_ADDRESS_ID]
        FROM [ADDRESS] AS [Extent4]
        WHERE [Extent1].[CONTACT_ADDRESS_ID] = [Extent4].[CONTACT_ADDRESS_ID] ) AS [Project2] ON 1 = 1
    WHERE [Project1].[CONTACT_ADDRESS_ID] = [Extent2].[CONTACT_ADDRESS_ID]
)

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在我看来，这是非常低效的，迫使你进入身份证途径。为什么它要做两次左连接，更不用说一次了？？？

我不能说出ADO.NET团队的想法或代码。尽管如此，我看到了两个可能的问题：

很可能，

ContractAddresses

中的基础表中的

Id

字段，或者可能仅在实体模型中，无法定义为主键。我有点怀疑这是个问题，但值得仔细检查

实际上，

equals

关键字可能没有很好的方法来比较联接中两个对象之间的相等性。在快速的web搜索中，我没有找到

equals

用于比较的确切内容，但让我相信

equals

和

GetHashCode

方法都涉及其中（即使不涉及组合键）。如果您只是使用默认的

object.Equals

继承方法，Linq提供程序必须以某种方式找出引用等式，我想这可能会导致一些奇怪的结果

---

不过，我确实喜欢@Craig Stuntz在评论中提出的解决方案。此外，您可能需要为较长的查询获取一个执行计划，以查看它是否真的像看上去的那样糟糕；查询优化器可能比代码所显示的做得更好。

最后，对于我来说，EF仍然缺乏在广阔的世界中执行所需的成熟度和功能。因此，我放弃了它，转而选择了NHibernate，它可以生成简单漂亮且经过优化的SQL。

即使这个问题非常好，我们在这里也很难回答。这是ADO.NET团队的问题。如果ADO.NET团队没有订阅这些关键字，他们需要在某人的十字准线中：）FWIW，我不会写任何一个。我将从companyDirector中的companyDirector从companyDirector中的contactAddress写入

。ContactAddresses选择新的{companyDirector，contactAddress}

，它应该可以正常工作，并且不需要了解DB架构即可写入。

FROM  [COMPANY] AS [Extent1]
INNER JOIN [ADDRESS] AS [Extent2] ON  EXISTS (SELECT 
    1 AS [C1]
    FROM    ( SELECT 1 AS X ) AS [SingleRowTable1]
    LEFT OUTER JOIN  (SELECT 
        [Extent3].[CONTACT_ADDRESS_ID] AS [CONTACT_ADDRESS_ID]
        FROM [ADDRESS] AS [Extent3]
        WHERE [Extent1].[CONTACT_ADDRESS_ID] = [Extent3].[CONTACT_ADDRESS_ID] ) AS [Project1] ON 1 = 1
    LEFT OUTER JOIN  (SELECT 
        [Extent4].[CONTACT_ADDRESS_ID] AS [CONTACT_ADDRESS_ID]
        FROM [ADDRESS] AS [Extent4]
        WHERE [Extent1].[CONTACT_ADDRESS_ID] = [Extent4].[CONTACT_ADDRESS_ID] ) AS [Project2] ON 1 = 1
    WHERE [Project1].[CONTACT_ADDRESS_ID] = [Extent2].[CONTACT_ADDRESS_ID]
)