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Multiple left unions in LINQ

I am trying to convert a SQL query to LINQ and I am having trouble setting the syntax correctly. My original (working) SQL query:

 SELECT a.PersonnelNumber,
    a.LastName,
    a.FirstName,
    a.MiddleInitial,
    b.Title,
    b.Division,
    b.Section,
    b.Unit,
    d.PersonnelNumber AS SupervisorPersonnelNumber
FROM Person a
     JOIN Position b ON a.PositionID = b.PositionID
     LEFT JOIN Position c ON b.SupervisorPositionID = c.PositionID
     LEFT JOIN Person d ON c.PositionID = d.PositionID

I converted this to the following LINQ:

var query = from a in ctx.People
            from b in ctx.Positions.Where(b => a.PositionID == b.PositionID)
            from c in ctx.Positions.Where(c => b.SupervisorPositionID == c.PositionID).DefaultIfEmpty()
            from d in ctx.People.Where(d => c.PositionID == d.PositionID).DefaultIfEmpty()
            select new { 
                            a.PersonnelNumber, 
                            a.LastName, 
                            a.FirstName, 
                            a.MiddleInitial, 
                            b.Title, 
                            b.Division, 
                            b.Section, 
                            b.Unit, 
                            SupervisorPersonnelNumber = d.PersonnelNumber
                       };

This returned more results than I expected (20000+ instead of ~ 1100), so I looked at the generated SQL:

SELECT 
[Extent2].[PositionID] AS [PositionID], 
[Extent1].[PersonnelNumber] AS [PersonnelNumber], 
[Extent1].[LastName] AS [LastName], 
[Extent1].[FirstName] AS [FirstName], 
[Extent1].[MiddleInitial] AS [MiddleInitial], 
[Extent2].[Title] AS [Title], 
[Extent2].[Division] AS [Division], 
[Extent2].[Section] AS [Section], 
[Extent2].[Unit] AS [Unit], 
[Extent4].[PersonnelNumber] AS [PersonnelNumber1]
FROM    [dbo].[Person] AS [Extent1]
INNER JOIN [dbo].[Position] AS [Extent2] ON [Extent1].[PositionID] = [Extent2].[PositionID]
LEFT OUTER JOIN [dbo].[Position] AS [Extent3] ON [Extent2].[SupervisorPositionID] = [Extent3].[PositionID]
LEFT OUTER JOIN [dbo].[Person] AS [Extent4] ON ([Extent3].[PositionID] = [Extent4].[PositionID]) OR (([Extent3].[PositionID] IS NULL) AND ([Extent4].[PositionID] IS NULL))

The last line of this is what is causing my problem:

LEFT OUTER JOIN [dbo].[Person] AS [Extent4] ON ([Extent3].[PositionID] = [Extent4].[PositionID]) OR (([Extent3].[PositionID] IS NULL) AND ([Extent4].[PositionID] IS NULL))

I wasn't sure why the additional clause was added OR

and removing it returned the desired results.

In case that helps, the table Position

has (in fact, though not enforced) a 1: 1 s ratio Person

and Position

is relevant to itself: PositionID

is FK beforeSupervisorPositionID 

CREATE TABLE [dbo].[Position](
[PositionID] [int] IDENTITY(1,1) NOT NULL,
[PositionNumber] [varchar](8) NULL,
[Title] [varchar](40) NULL,
[Division] [varchar](40) NULL,
[Section] [varchar](40) NULL,
[Unit] [varchar](40) NULL,
[SupervisorPositionID] [int] NULL,
)

CREATE TABLE [dbo].[Person](
[PersonID] [int] IDENTITY(1,1) NOT NULL,
[PersonnelNumber] [varchar](8) NOT NULL,
[LastName] [varchar](40) NULL,
[FirstName] [varchar](40) NULL,
[MiddleInitial] [char](1) NULL,
[PositionID] [int] NULL,
)

Why is it being OR (([Extent3].[PositionID] IS NULL) AND ([Extent4].[PositionID] IS NULL))

added to the end of this line and what can I do to fix it?

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2 answers


You need to change the setting to your DbContext. UseDatabaseNullSemantics property and you need to set it to false



context.Configuration.UseDatabaseNullSemantics = false;
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I don't have the tools at the moment to test if this produces the desired result, but I think it should be close enough:

            from a in Persons
            join b in Positions on a.PositionID equals b.PositionID
            join c in Positions on b.SupervisorPositionID equals c.PositionID into SupervisorsPositions
                from c in SupervisorsPositions.DefaultIfEmpty()
            join d in Persons on c.PositionID equals d.PositionID into PersonalNumbers
                from d in PersonalNumbers.DefaultIfEmpty()
            select new { 
                            a.PersonnelNumber, 
                            a.LastName, 
                            a.FirstName, 
                            a.MiddleInitial, 
                            b.Title, 
                            b.Division, 
                            b.Section, 
                            b.Unit, 
                            SupervisorPersonnelNumber = d.PersonnelNumber
                       }

This query will produce:



SELECT [t0].[PersonnelNumber], [t0].[LastName], [t0].[FirstName], [t0].[MiddleInitial], [t1].[Title], [t1].[Division], [t1].[Section], [t1].[Unit], [t3].[PersonnelNumber] AS [SupervisorPersonnelNumber]
FROM [Person] AS [t0]
INNER JOIN [Position] AS [t1] ON [t0].[PositionID] = ([t1].[PositionID])
LEFT OUTER JOIN [Position] AS [t2] ON [t1].[SupervisorPositionID] = ([t2].[PositionID])
LEFT OUTER JOIN [Person] AS [t3] ON ([t2].[PositionID]) = [t3].[PositionID]

From T-SQL Language Specification LEFT OUTER JOIN equals LEFT JOIN. INNER JOIN equals JOIN.

Thus, the query produces the result you want. See details for more information on join types in T-SQL.

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