Linq to Entities How to return all records from Parent

left-joinlinq-to-entities

Employee table

Employee_ID int
Employee_Name varchar(50)

Sales table:

Sales_ID int
Employee_ID int
Sale_Amount money

Standard SQL select

Select
*
From
Employee emp
left outer join
Sales s
on
s.Employee_ID = emp.Employee_ID

Standard SQL Results (The exact results I want using Linq to Entites)

1 Emp1 1 1 150.00
1 Emp1 2 1 500.00
2 Emp2 3 2 250.00
3 Emp3 NULL NULL NULL
4 Emp4 NULL NULL NULL
5 Emp5 4 5 700.00

Now to tackle Linq To Entities

            Dim query = From emp In entiites.Employee _
                    From sales In emp.Sales _
                    Select _
                        emp, _
                        sales

Linq To Entities Result (Where is Employee_ID 3 and 4)

1: Emp1: 150.0000
1: Emp1: 500.0000
2: Emp2: 250.0000
5: Emp5: 700.0000

Try it with Linq to Entities with a left outer join:

            Dim query = From emp In entiites.Employee _
                    Group Join sales In entiites.Sales _
                    On emp.Employee_ID Equals sales.Employee.Employee_ID _
                    Into sales_grp = Group _
                    From Sel_SalesGrp In sales_grp.DefaultIfEmpty() _
                    Select _
                        emp, _
                        Sel_SalesGrp

Then I get this error using DefaultIfEmpty:

LINQ to Entities does not recognize the method 'System.Collections.Generic.IEnumerable1[m12Model.Sales] DefaultIfEmpty[Sales](System.Collections.Generic.IEnumerable1[m12Model.Sales])' method, and this method cannot be translated into a store expression.

Linq to Entites: does not support DefaultIfEmpty().

Best Answer

You need to perform a left join by using the "Into" and "DefaultIfEmpty()" keywords:

Dim query = From emp In entities.Employee _
        Join sal In entities.Sales On emp.Employee_ID Equals sal.Employee_ID Into LeftJoinSal _
        From ljs In LeftJoinSal.DefaultIfEmpty() _
        Select _
            emp, _
            ljs

This site shows another simple example: GeeksWithBlogs. Though it is in C#.

The key is that you need to join into a new name to prevent it from filtering the results from your first table (which hides the rows that don't join), then select from that using the DefaultIfEmpty() function, which provides a default (null) value in the cases where there is no join.