Question

我有以下课程，例如

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; }
}

我有LazyLoadingEnabled = false和ProxyCreationEnabled = false

当我这样做时

var team = db.Teams.Where(x => x.TeamId == 1).Include(x=>x.TeamFeeds);

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，所以我可能不会理解某些事情。

由于

Answer 1

首先，当INNER JOIN的团队没有Feed时，使用TeamID = 1的查询不会返回任何内容。这里必须使用LEFT JOIN，这就是EF在ineer SELECT中所做的事情：

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

关注可怕的行只是为了让EF知道，如果指定了JOIN的左侧网站 - 如果团队没有Feed且0则会返回1如果找到了至少一个Feed。

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

外部SELECT语句只是将DataBase列名称映射到EF模型属性名称，并ORDER BY。

希望这足以说明为什么EF生成了这样的一个冗长的SQL语句。或者更确切地说，为什么这个陈述看起来很可怕，但实际上并不是简单的SELECT * FROM Teams JOIN Feeds。

使用Sub Select而不是INNER JOIN的实体框架

1 个答案: