LINQ to SQL Left Outer Join

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Is this query equivalent to a LEFT OUTER join?

//assuming that I have a parameter named 'invoiceId' of type int
from c in SupportCases
let invoice = c.Invoices.FirstOrDefault(i=> i.Id == invoiceId)
where (invoiceId == 0 || invoice != null)    
select new 
{
      Id = c.Id
      , InvoiceId = invoice == null ? 0 : invoice.Id
}

Not quite - since each "left" row in a left-outer-join will match 0-n "right" rows (in the second table), where-as yours matches only 0-1. To do a left outer join, you need SelectMany and DefaultIfEmpty, for example:

var query = from c in db.Customers
            join o in db.Orders
               on c.CustomerID equals o.CustomerID into sr
            from x in sr.DefaultIfEmpty()
            select new {
               CustomerID= c.CustomerID, ContactName=c.ContactName,
               OrderID = x.OrderID == null ? -1 : x.OrderID};   

(or via the extension methods)

Perform left outer joins (LINQ in C#), Not quite - since each "left" row in a left-outer-join will match 0-n "right" rows (in the second table), where-as yours matches only 0-1. To do a left  A left outer join is a join in which each element of the first collection is returned, regardless of whether it has any correlated elements in the second collection. You can use LINQ to perform a left outer join by calling the DefaultIfEmpty method on the results of a group join.

You don't need the into statements:

var query = 
    from customer in dc.Customers
    from order in dc.Orders
         .Where(o => customer.CustomerId == o.CustomerId)
         .DefaultIfEmpty()
    select new { Customer = customer, Order = order } 
    //Order will be null if the left join is null

And yes, the query above does indeed create a LEFT OUTER join.

Link to a similar question that handles multiple left joins: Linq to Sql: Multiple left outer joins

LINQ to SQL Left Outer Join, This article explains various types of joins in LINQ query. Learn Inner join, Outer Join, Left Outer join, Right Outer join, Full outer join, Cross Join  LINQ to SQL Left Outer Join In LINQ to SQL, the LEFT JOIN is used to return all the records or rows from the left table and matching records from the right table. If there are no columns matching in the right table, then it will return only left table records.

Public Sub LinqToSqlJoin07()
Dim q = From e In db.Employees _
        Group Join o In db.Orders On e Equals o.Employee Into ords = Group _
        From o In ords.DefaultIfEmpty _
        Select New With {e.FirstName, e.LastName, .Order = o}

ObjectDumper.Write(q) End Sub

Check http://msdn.microsoft.com/en-us/vbasic/bb737929.aspx

SQL Join in LINQ (LINQ to Entity / LINQ to SQL), In this example we will write LINQ Left outer join on tblEmployees and tblDepartments table. Input tables. Department table. departments. Employees table. Left Outer Join in Linq using Method Syntax: In order to implement Left Outer Join in Linq using Method Syntax we need to use the GroupJoin() method along with SelectMany() and DefaultIfEmpty() methods. So, let us rewrite the previous example using Method Syntax as shown below. It will give you the same output as the previous example.

I found 1 solution. if want to translate this kind of SQL (left join) into Linq Entity...

SQL:

SELECT * FROM [JOBBOOKING] AS [t0]
LEFT OUTER JOIN [REFTABLE] AS [t1] ON ([t0].[trxtype] = [t1].[code])
                                  AND ([t1]. [reftype] = "TRX")

LINQ:

from job in JOBBOOKINGs
join r in (from r1 in REFTABLEs where r1.Reftype=="TRX" select r1) 
          on job.Trxtype equals r.Code into join1
from j in join1.DefaultIfEmpty()
select new
{
   //cols...
}

LINQ to SQL Left Outer Join, Before calling DefaultIfEmpty(), you need to introduce your join condition. You should just use extension method syntax like this: from p in  LINQ only supports left outer joins. If you want a right outer join, you need to flip the logic of your query to turn it into a left outer join. In order to use the DefaultIfEmpty, you typically need to push the joined set into a temporary value first and then select from it using the DefaultIfEmpty method:

I'd like to add one more thing. In LINQ to SQL if your DB is properly built and your tables are related through foreign key constraints, then you do not need to do a join at all.

Using LINQPad I created the following LINQ query:

//Querying from both the CustomerInfo table and OrderInfo table
from cust in CustomerInfo
where cust.CustomerID == 123456
select new {cust, cust.OrderInfo}

Which was translated to the (slightly truncated) query below

 -- Region Parameters
 DECLARE @p0 Int = 123456
-- EndRegion
SELECT [t0].[CustomerID], [t0].[AlternateCustomerID],  [t1].[OrderID], [t1].[OnlineOrderID], (
    SELECT COUNT(*)
    FROM [OrderInfo] AS [t2]
    WHERE [t2].[CustomerID] = [t0].[CustomerID]
    ) AS [value]
FROM [CustomerInfo] AS [t0]
LEFT OUTER JOIN [OrderInfo] AS [t1] ON [t1].[CustomerID] = [t0].[CustomerID]
WHERE [t0].[CustomerID] = @p0
ORDER BY [t0].[CustomerID], [t1].[OrderID]

Notice the LEFT OUTER JOIN above.

LINQ to SQL - Left Outer Join with multiple join conditions, The Left Outer Join in Linq is a join in which each data from the first data source is returned irrespective of whether it has any correlated data in 2nd. Looks perfect, but the SQL generated would always be an INNER JOIN, not a LEFT OUTER JOIN. The Final Solution. I tried modifying the above code with many different combinations of DefaultIfEmpty method calls and NULL comparisons, but could not persuade the Entity Framework to generate a LEFT OUTER JOIN.

Left Outer Join in Linq With Examples, Learn with examples in C#: LINQ to SQL on JOIN operations. The examples include INNER JOIN, OUTER JOIN (left, right, and full), and CROSS JOIN. LINQ to SQL Left Outer Join . In this example we will write LINQ Left outer join on tblEmployees and tblDepartments table. Input tables

LINQ to SQL: JOIN Table Examples (C#) - Querychat, Please modify your SQL statement as below: select * from [AvailableStocks] a LEFT OUTER JOIN (select * from UserSelectedStocks where  I have this query in SQL, and I want it to implement it in LINQ using Entity Framework, but how can I apply multiple tables left outer joins?. SELECT d.bookingid, d.labid, d.processid, p.prid, p.prno, d.DestinationBranchID, d.SendStatus FROM dc_tpatient_bookingd d LEFT OUTER JOIN dc_tpatient_bookingm m ON d.bookingid = m.bookingid LEFT OUTER JOIN dc_tpatient p ON p.prid = m.prid LEFT OUTER

Left Join in SQL and LINQ, In this post, we will see an example of how to do a Left Outer Join in LINQ and C#​. In a previous post, we saw how to do an Inner join in C# and LINQ where each Actually I expected same results like a "SQL-Left-Join", that will join table  LEFT JOIN or LEFT OUTER JOIN LEFT JOIN returns all records or rows from the left table and from right table returns only matched records. If there are no columns matching in the right table, it returns NULL values. In LINQ to achieve LEFT JOIN behavior, it is mandatory to use "INTO" keyword and "DefaultIfEmpty()" method.

Comments
  • Can someone explain how this crazy syntax works? I fail to see how any of those keywords magically makes it a left join. What does the "into sr" do? Linq frustrates me sometimes :)
  • @JoePhillips I have plenty of SQL experience but trying to learn LINQ is like wading through mud. I agree it is absolutely crazy.
  • @marc-gravell:Could you help me in solving my sql query to linq conversion : stackoverflow.com/questions/28367941/…
  • @VishalIPatil why do you want to convert from SQL to LINQ? SQL works just fine and is far more predictable and efficient...
  • @VishalIPatil so... why do that? Just about every LINQ tool includes the ability to run hand-written SQL. Why not just do that?
  • While I know that @Marc Gravvel's answer does work, I really prefer this method because IMO it feels more in line with what a left join should look like.
  • Excellent answer. Looking for more than 5 hours of google search. This is the only way resulting SQL will have left join in it.
  • THANK YOU soooo much....I was searching for a solution for this all afternoon and your code nailed it (and feels natural to boot). Wish I could upvote this several times.
  • @Jim thanks :-) I'm glad devs are still getting mileage out of this answer. I completely agree that the DefaultIfEmpty() feels a lot more natural than using the into statements.
  • Just a note for anyone else who finds this like I just did, this results in a LEFT OUTER JOIN inside a CROSS APPLY, which means you will get duplicates if there are multiple matches on the right-hand side of the join. Marc Gravell's solution, while not as "pretty" gave me the proper SQL output and result set that I was looking for.
  • Nice try but it looks like the OP is using c#. The VB syntax is oddly different.
  • See this comment, Linq-to-SQL entities don't support DefaultIfEmpty.