使用Sub Select而不是INNER JOIN的实体框架

Question

I have the below classes such as

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; }
}

I have LazyLoadingEnabled = false and ProxyCreationEnabled = false

When I do

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

The SQL generated by EF looks like:

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

I don't understand why it hasn't used SQL that I would use and created a long winded SQL statement.

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

I'm new to EF so it may be I'm not be understanding something.

Thanks

Answer 1

First of all, your query using INNER JOIN won't return anything, when Team with TeamID = 1 has no feeds. LEFT JOIN has to be used here, and that's what EF did in ineer SELECT :

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

Following scary line is just to let EF know, if left site of the JOIN was specified - it returns 0 if Team has no Feeds and 1 if at lease one Feed was found.

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

And outer SELECT statement just maps DataBase columns names to EF model properties names and does ORDER BY .

Hope it's enough of explenation why EF generated such a long winded SQL statement . Or rather why this statement only looks scary, but really is not much more then your simple SELECT * FROM Teams JOIN Feeds .