C# 使用子选择非内部联接的实体框架

我有以下课程，如

public class Team
{
    [Key]
    public virtual Int32 TeamId { get; set; }
    [Required]
    public virtual String Name { get; set; }
    public virtual String Description { get; set; }
    public virtual ICollection<TeamFeed> TeamFeeds { get; set; }
}

public class TeamFeed
{       
    public Int32 TeamFeedId { get; set; }
    [Required]
    public Int32 TeamId { get; set; }
    public virtual bool IsEnabled { get; set; }
    public virtual Team Team { get; set; }
}

EF生成的SQL如下所示：

SELECT 
[Project1].[TeamId] AS [TeamId], 
[Project1].[Name] AS [Name], 
[Project1].[Description] AS [Description], 
[Project1].[C1] AS [C1], 
[Project1].[TeamFeedId] AS [TeamFeedId], 
[Project1].[TeamId1] AS [TeamId1], 
[Project1].[IsEnabled] AS [IsEnabled], 
FROM ( SELECT 
    [Extent1].[TeamId] AS [TeamId], 
    [Extent1].[Name] AS [Name], 
    [Extent1].[Description] AS [Description],  
    [Extent2].[TeamFeedId] AS [TeamFeedId], 
    [Extent2].[TeamId] AS [TeamId1], 
    [Extent2].[IsEnabled] AS [IsEnabled],  
    CASE WHEN ([Extent2].[TeamFeedId] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
    FROM  [dbo].[Teams] AS [Extent1]
    LEFT OUTER JOIN [dbo].[TeamFeeds] AS [Extent2] ON [Extent1].[TeamId] = [Extent2].[TeamId]
    WHERE 1 = [Extent1].[TeamId]
)  AS [Project1]
ORDER BY [Project1].[TeamId] ASC, [Project1].[C1] ASC

我不明白为什么它没有使用我会使用的SQL，并创建了一个冗长的SQL语句

SELECT 
* --use all just for clarity in the example
FROM TEAMS T

INNER JOIN TEAMFEEDS TF
ON T.TEAMID = TF.TEAMID

ORDER BY 
T.TEAMID

我是EF的新手，所以我可能不了解一些东西

---

谢谢

首先，当

TeamID=1

的团队没有提要时，使用

内部联接的查询将不会返回任何内容<这里必须使用code>LEFT JOIN

，这就是EF在ineer

SELECT

中所做的：

SELECT 
    *
    FROM  [dbo].[Teams] AS [Extent1]
    LEFT OUTER JOIN [dbo].[TeamFeeds] AS [Extent2] ON [Extent1].[TeamId] = [Extent2].[TeamId]
    WHERE 1 = [Extent1].[TeamId]

下面这行可怕的代码只是为了让EF知道，若指定了

JOIN

的左侧站点，若团队并没有提要，则返回

0

，若至少找到一个提要，则返回

1

CASE WHEN ([Extent2].[TeamFeedId] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]

而outer

SELECT

语句只是将数据库列名称映射到EF模型属性名称，并按排序

---

希望这足以解释为什么EF生成了如此冗长的SQL语句。或者更确切地说，为什么这句话看起来很吓人，但实际上并不比简单的

SELECT*fromteams-JOIN-feed

实体框架创建的查询更详细，但它们的性能通常与较短的查询一样好（有时甚至更好）。当然，EF通常比较保守，生成的代码处理您可能不关心的边缘情况。另外，EF做你要求它做的事情，而不一定是你想让它做的事情，所以有时候它会创建糟糕的代码，因为它正试图做你让它做的事情，但如果不这样做，它就不能。

CASE WHEN ([Extent2].[TeamFeedId] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]