LEFT JOIN in LINQ to entities?

Question

I'm trying out LINQ to entities.

I have a problem with the following: I want it to do this:

SELECT 
     T_Benutzer.BE_User
    ,T_Benutzer_Benutzergruppen.BEBG_BE
FROM T_Benutzer

LEFT JOIN T_Benutzer_Benutzergruppen
    ON T_Benutzer_Benutzergruppen.BEBG_BE = T_Benutzer.BE_ID 

the closest thing I've come to is this:

        var lol = (
            from u in Repo.T_Benutzer

            //where u.BE_ID == 1
            from o in Repo.T_Benutzer_Benutzergruppen.DefaultIfEmpty()
                // on u.BE_ID equals o.BEBG_BE

            where (u.BE_ID == o.BEBG_BE || o.BEBG_BE == null)

            //join bg in Repo.T_Benutzergruppen.DefaultIfEmpty()
            //    on o.BEBG_BG equals bg.ID

            //where bg.ID == 899 

            orderby
                u.BE_Name ascending
                //, bg.Name descending

            //select u 
            select new
            {
                 u.BE_User
                ,o.BEBG_BG
                //, bg.Name 
            }
         ).ToList();

But this generates the same results as an inner join, and not a left join.
Moreover, it creates this completely crazy SQL:

SELECT 
     [Extent1].[BE_ID] AS [BE_ID]
    ,[Extent1].[BE_User] AS [BE_User]
    ,[Join1].[BEBG_BG] AS [BEBG_BG]
FROM  [dbo].[T_Benutzer] AS [Extent1]

CROSS JOIN  
(
    SELECT 
         [Extent2].[BEBG_BE] AS [BEBG_BE]
        ,[Extent2].[BEBG_BG] AS [BEBG_BG]
    FROM ( SELECT 1 AS X ) AS [SingleRowTable1]
    LEFT OUTER JOIN [dbo].[T_Benutzer_Benutzergruppen] AS [Extent2] 
        ON 1 = 1 
) AS [Join1]

WHERE [Extent1].[BE_ID] = [Join1].[BEBG_BE] 
OR [Join1].[BEBG_BE] IS NULL

ORDER BY [Extent1].[BE_Name] ASC

How can I do a left join in LINQ-2-entities in a way where another person can still understand what's being done in that code ?

and most-preferably where the generated SQL looks like:

SELECT 
     T_Benutzer.BE_User
    ,T_Benutzer_Benutzergruppen.BEBG_BE
FROM T_Benutzer

LEFT JOIN T_Benutzer_Benutzergruppen
    ON T_Benutzer_Benutzergruppen.BEBG_BE = T_Benutzer.BE_ID 

Answer 1

Ah, got it myselfs.
The quirks and quarks of LINQ-2-entities.
This looks most understandable:

var query2 = (
    from users in Repo.T_Benutzer
    from mappings in Repo.T_Benutzer_Benutzergruppen
        .Where(mapping => mapping.BEBG_BE == users.BE_ID).DefaultIfEmpty()
    from groups in Repo.T_Benutzergruppen
        .Where(gruppe => gruppe.ID == mappings.BEBG_BG).DefaultIfEmpty()
    //where users.BE_Name.Contains(keyword)
    // //|| mappings.BEBG_BE.Equals(666)  
    //|| mappings.BEBG_BE == 666 
    //|| groups.Name.Contains(keyword)

    select new
    {
         UserId = users.BE_ID
        ,UserName = users.BE_User
        ,UserGroupId = mappings.BEBG_BG
        ,GroupName = groups.Name
    }

);


var xy = (query2).ToList();

Remove the .DefaultIfEmpty(), and you get an inner join.
That was what I was looking for.

Answer 2

You can read an article i have written for joins in LINQ here

var query = 
from  u in Repo.T_Benutzer
join bg in Repo.T_Benutzer_Benutzergruppen
    on u.BE_ID equals bg.BEBG_BE
into temp
from j in temp.DefaultIfEmpty()
select new
{
    BE_User = u.BE_User,
    BEBG_BG = (int?)j.BEBG_BG// == null ? -1 : j.BEBG_BG
            //, bg.Name 
}

The following is the equivalent using extension methods:

var query = 
Repo.T_Benutzer
.GroupJoin
(
    Repo.T_Benutzer_Benutzergruppen,
    x=>x.BE_ID,
    x=>x.BEBG_BE,
    (o,i)=>new {o,i}
)
.SelectMany
(
    x => x.i.DefaultIfEmpty(),
    (o,i) => new
    {
        BE_User = o.o.BE_User,
        BEBG_BG = (int?)i.BEBG_BG
    }
);

Answer 3

May be I come later to answer but right now I'm facing with this... if helps there are one more solution (the way i solved it).

    var query2 = (
    from users in Repo.T_Benutzer
    join mappings in Repo.T_Benutzer_Benutzergruppen on mappings.BEBG_BE equals users.BE_ID into tmpMapp
    join groups in Repo.T_Benutzergruppen on groups.ID equals mappings.BEBG_BG into tmpGroups
    from mappings in tmpMapp.DefaultIfEmpty()
    from groups in tmpGroups.DefaultIfEmpty()
    select new
    {
         UserId = users.BE_ID
        ,UserName = users.BE_User
        ,UserGroupId = mappings.BEBG_BG
        ,GroupName = groups.Name
    }

);

By the way, I tried using the Stefan Steiger code which also helps but it was slower as hell.

Answer 4

Easy way is to use Let keyword. This works for me.

from AItem in Db.A
Let BItem = Db.B.Where(x => x.id == AItem.id ).FirstOrDefault() 
Where SomeCondition
Select new YourViewModel
{
    X1 = AItem.a,
    X2 = AItem.b,
    X3 = BItem.c
}

This is a simulation of Left Join. If each item in B table not match to A item , BItem return null

Answer 5

You can use this not only in entities but also store procedure or other data source:

var customer = (from cus in _billingCommonservice.BillingUnit.CustomerRepository.GetAll()  
                          join man in _billingCommonservice.BillingUnit.FunctionRepository.ManagersCustomerValue()  
                          on cus.CustomerID equals man.CustomerID  
                          // start left join  
                          into a  
                          from b in a.DefaultIfEmpty(new DJBL_uspGetAllManagerCustomer_Result() )  
                          select new { cus.MobileNo1,b.ActiveStatus });