Question

I have a LINQ query, which for some reason is generating an extra/duplicate INNER JOIN. This is causing the query to not return the expected output. If I manually comment that extra JOIN from the generated SQL, then I get seemingly correct output.

Can you detect what I might have done in this LINQ to have caused this extra JOIN?

Thanks.

Here is my approx LINQ

predicate=predicate.And(condition1);
predicate1=predicate1.And(condition2);
predicate1=predicate1.And(condition3);
predicate2=predicate2.Or(predicate1);
predicate=predicate.And(predicate2);
var ids = context.Code.Where(predicate); 
            var rs = from r in ids 
                     group r by r.PersonID into g 
                     let matchcount=g.Select(p => p.phonenumbers.PhoneNum).Distinct().Count() 
                     where matchcount ==2 
                     select new 
                  { 
                      personid = g.Key 
                  };

and here is the generated SQL (the duplicate join is [t7])

Declare @p1 VarChar(10)='Home'
Declare @p2 VarChar(10)='111'
Declare @p3 VarChar(10)='Office'
Declare @p4 VarChar(10)='222'
Declare @p5 int=2

SELECT [t9].[PersonID] AS [pid]
FROM (
    SELECT [t3].[PersonID], (
        SELECT COUNT(*)
        FROM (
            SELECT DISTINCT [t7].[PhoneValue]
            FROM [dbo].[Person] AS [t4]
            INNER JOIN [dbo].[PersonPhoneNumber] AS [t5] ON [t5].[PersonID] = [t4].[PersonID]
            INNER JOIN [dbo].[CodeMaster] AS [t6] ON [t6].[Code] = [t5].[PhoneType]
            INNER JOIN [dbo].[PersonPhoneNumber] AS [t7] ON [t7].[PersonID] = [t4].[PersonID]
            WHERE ([t3].[PersonID] = [t4].[PersonID]) AND ([t6].[Enumeration] = @p0) AND ((([t6].[CodeDescription] = @p1) AND ([t5].[PhoneValue] = @p2)) OR (([t6].[CodeDescription] = @p3) AND ([t5].[PhoneValue] = @p4)))
            ) AS [t8]
        ) AS [value]
    FROM (
        SELECT [t0].[PersonID]
        FROM [dbo].[Person] AS [t0]
        INNER JOIN [dbo].[PersonPhoneNumber] AS [t1] ON [t1].[PersonID] = [t0].[PersonID]
        INNER JOIN [dbo].[CodeMaster] AS [t2] ON [t2].[Code] = [t1].[PhoneType]
        WHERE ([t2].[Enumeration] = @p0) AND ((([t2].[CodeDescription] = @p1) AND ([t1].[PhoneValue] = @p2)) OR (([t2].[CodeDescription] = @p3) AND ([t1].[PhoneValue] = @p4)))
        GROUP BY [t0].[PersonID]
        ) AS [t3]
    ) AS [t9]
WHERE [t9].[value] = @p5

Answer 1

They aren’t being duplicated. You are asking for two different values from the data source.

let matchcount=g.Select(p => p.phonenumbers.PhoneNum).Distinct().Count()

is causing

        SELECT COUNT(*) 
        FROM ( 
            SELECT DISTINCT [t7].[PhoneValue] 
            FROM [dbo].[Person] AS [t4] 
            INNER JOIN [dbo].[PersonPhoneNumber] AS [t5] ON [t5].[PersonID] = [t4].[PersonID] 
            INNER JOIN [dbo].[CodeMaster] AS [t6] ON [t6].[Code] = [t5].[PhoneType] 
            INNER JOIN [dbo].[PersonPhoneNumber] AS [t7] ON [t7].[PersonID] = [t4].[PersonID] 
            WHERE ([t3].[PersonID] = [t4].[PersonID]) AND ([t6].[Enumeration] = @p0) AND ((([t6].[CodeDescription] = @p1) AND ([t5].[PhoneValue] = @p2)) OR (([t6].[CodeDescription] = @p3) AND ([t5].[PhoneValue] = @p4))) 
            ) AS [t8] 

and

                     from r in ids   
                     group r by r.PersonID into g 

is causing

    SELECT [t0].[PersonID]    
    FROM [dbo].[Person] AS [t0]    
    INNER JOIN [dbo].[PersonPhoneNumber] AS [t1] ON [t1].[PersonID] = [t0].[PersonID]    
    INNER JOIN [dbo].[CodeMaster] AS [t2] ON [t2].[Code] = [t1].[PhoneType]    
    WHERE ([t2].[Enumeration] = @p0) AND ((([t2].[CodeDescription] = @p1) AND ([t1].[PhoneValue] = @p2)) OR (([t2].[CodeDescription] = @p3) AND ([t1].[PhoneValue] = @p4)))    
    GROUP BY [t0].[PersonID]    
    ) AS [t3]   

as for the INNER JOINS, the reason you are getting them is because of the relationship between those tables. For instance Person is 1..1 with PersonPhoneNumber (or 1..*). In either case I assume PersonID on PersonPhoneNumber is an FK and a PK value. So in that case the data source has to go out to that external table to see if the value for the PersonPhoneNumber navigation property actually exists. It does this by performing an INNER JOIN on that table.