LEFT OUTER JOIN in LINQ

As stated in "Perform left outer joins":

var q =
    from c in categories
    join p in products on c.Category equals p.Category into ps
    from p in ps.DefaultIfEmpty()
    select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName };

---

If a database driven LINQ provider is used, a significantly more readable left outer join can be written as such:

from maintable in Repo.T_Whatever 
from xxx in Repo.T_ANY_TABLE.Where(join condition).DefaultIfEmpty()

If you omit the DefaultIfEmpty() you will have an inner join.

Take the accepted answer:

  from c in categories
    join p in products on c equals p.Category into ps
    from p in ps.DefaultIfEmpty()

This syntax is very confusing, and it's not clear how it works when you want to left join MULTIPLE tables.

Note
It should be noted that from alias in Repo.whatever.Where(condition).DefaultIfEmpty() is the same as an outer-apply/left-join-lateral, which any (decent) database-optimizer is perfectly capable of translating into a left join, as long as you don't introduce per-row-values (aka an actual outer apply). Don't do this in Linq-2-Objects (because there's no DB-optimizer when you use Linq-to-Objects).

Detailed Example

var query2 = (
    from users in Repo.T_User
    from mappings in Repo.T_User_Group
         .Where(mapping => mapping.USRGRP_USR == users.USR_ID)
         .DefaultIfEmpty() // <== makes join left join
    from groups in Repo.T_Group
         .Where(gruppe => gruppe.GRP_ID == mappings.USRGRP_GRP)
         .DefaultIfEmpty() // <== makes join left join

    // where users.USR_Name.Contains(keyword)
    // || mappings.USRGRP_USR.Equals(666)  
    // || mappings.USRGRP_USR == 666 
    // || groups.Name.Contains(keyword)

    select new
    {
         UserId = users.USR_ID
        ,UserName = users.USR_User
        ,UserGroupId = groups.ID
        ,GroupName = groups.Name
    }

);


var xy = (query2).ToList();

When used with LINQ 2 SQL it will translate nicely to the following very legible SQL query:

SELECT 
     users.USR_ID AS UserId 
    ,users.USR_User AS UserName 
    ,groups.ID AS UserGroupId 
    ,groups.Name AS GroupName 
FROM T_User AS users

LEFT JOIN T_User_Group AS mappings
   ON mappings.USRGRP_USR = users.USR_ID

LEFT JOIN T_Group AS groups
    ON groups.GRP_ID == mappings.USRGRP_GRP

Edit:

See also " Convert SQL Server query to Linq query " for a more complex example.

Also, If you're doing it in Linq-2-Objects (instead of Linq-2-SQL), you should do it the old-fashioned way (because LINQ to SQL translates this correctly to join operations, but over objects this method forces a full scan, and doesn't take advantage of index searches, whyever...):

    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
    }

);

---

Using lambda expression

db.Categories    
  .GroupJoin(db.Products,
      Category => Category.CategoryId,
      Product => Product.CategoryId,
      (x, y) => new { Category = x, Products = y })
  .SelectMany(
      xy => xy.Products.DefaultIfEmpty(),
      (x, y) => new { Category = x.Category, Product = y })
  .Select(s => new
  {
      CategoryName = s.Category.Name,     
      ProductName = s.Product.Name   
  });

---

Now as an extension method:

public static class LinqExt
{
    public static IEnumerable<TResult> LeftOuterJoin<TLeft, TRight, TKey, TResult>(this IEnumerable<TLeft> left, IEnumerable<TRight> right, Func<TLeft, TKey> leftKey, Func<TRight, TKey> rightKey,
        Func<TLeft, TRight, TResult> result)
    {
        return left.GroupJoin(right, leftKey, rightKey, (l, r) => new { l, r })
             .SelectMany(
                 o => o.r.DefaultIfEmpty(),
                 (l, r) => new { lft= l.l, rght = r })
             .Select(o => result.Invoke(o.lft, o.rght));
    }
}

Use like you would normally use join:

var contents = list.LeftOuterJoin(list2, 
             l => l.country, 
             r => r.name,
            (l, r) => new { count = l.Count(), l.country, l.reason, r.people })

Hope this saves you some time.

---

Take a look at this example. This query should work:

var leftFinal = from left in lefts
                join right in rights on left equals right.Left into leftRights
                from leftRight in leftRights.DefaultIfEmpty()
                select new { LeftId = left.Id, RightId = left.Key==leftRight.Key ? leftRight.Id : 0 };

---

An implementation of left outer join by extension methods could look like

public static IEnumerable<Result> LeftJoin<TOuter, TInner, TKey, Result>(
  this IEnumerable<TOuter> outer, IEnumerable<TInner> inner
  , Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector
  , Func<TOuter, TInner, Result> resultSelector, IEqualityComparer<TKey> comparer)
  {
    if (outer == null)
      throw new ArgumentException("outer");

    if (inner == null)
      throw new ArgumentException("inner");

    if (outerKeySelector == null)
      throw new ArgumentException("outerKeySelector");

    if (innerKeySelector == null)
      throw new ArgumentException("innerKeySelector");

    if (resultSelector == null)
      throw new ArgumentException("resultSelector");

    return LeftJoinImpl(outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer ?? EqualityComparer<TKey>.Default);
  }

  static IEnumerable<Result> LeftJoinImpl<TOuter, TInner, TKey, Result>(
      IEnumerable<TOuter> outer, IEnumerable<TInner> inner
      , Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector
      , Func<TOuter, TInner, Result> resultSelector, IEqualityComparer<TKey> comparer)
  {
    var innerLookup = inner.ToLookup(innerKeySelector, comparer);

    foreach (var outerElment in outer)
    {
      var outerKey = outerKeySelector(outerElment);
      var innerElements = innerLookup[outerKey];

      if (innerElements.Any())
        foreach (var innerElement in innerElements)
          yield return resultSelector(outerElment, innerElement);
      else
        yield return resultSelector(outerElment, default(TInner));
     }
   }

The resultselector then has to take care of the null elements. Fx.

   static void Main(string[] args)
   {
     var inner = new[] { Tuple.Create(1, "1"), Tuple.Create(2, "2"), Tuple.Create(3, "3") };
     var outer = new[] { Tuple.Create(1, "11"), Tuple.Create(2, "22") };

     var res = outer.LeftJoin(inner, item => item.Item1, item => item.Item1, (it1, it2) =>
     new { Key = it1.Item1, V1 = it1.Item2, V2 = it2 != null ? it2.Item2 : default(string) });

     foreach (var item in res)
       Console.WriteLine(string.Format("{0}, {1}, {2}", item.Key, item.V1, item.V2));
   }

---

take look at this example

class Person
{
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Phone { get; set; }
}

class Pet
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}

public static void LeftOuterJoinExample()
{
    Person magnus = new Person {ID = 1, FirstName = "Magnus", LastName = "Hedlund"};
    Person terry = new Person {ID = 2, FirstName = "Terry", LastName = "Adams"};
    Person charlotte = new Person {ID = 3, FirstName = "Charlotte", LastName = "Weiss"};
    Person arlene = new Person {ID = 4, FirstName = "Arlene", LastName = "Huff"};

    Pet barley = new Pet {Name = "Barley", Owner = terry};
    Pet boots = new Pet {Name = "Boots", Owner = terry};
    Pet whiskers = new Pet {Name = "Whiskers", Owner = charlotte};
    Pet bluemoon = new Pet {Name = "Blue Moon", Owner = terry};
    Pet daisy = new Pet {Name = "Daisy", Owner = magnus};

    // Create two lists.
    List<Person> people = new List<Person> {magnus, terry, charlotte, arlene};
    List<Pet> pets = new List<Pet> {barley, boots, whiskers, bluemoon, daisy};

    var query = from person in people
        where person.ID == 4
        join pet in pets on person equals pet.Owner  into personpets
        from petOrNull in personpets.DefaultIfEmpty()
        select new { Person=person, Pet = petOrNull}; 



    foreach (var v in query )
    {
        Console.WriteLine("{0,-15}{1}", v.Person.FirstName + ":", (v.Pet == null ? "Does not Exist" : v.Pet.Name));
    }
}

// This code produces the following output:
//
// Magnus:        Daisy
// Terry:         Barley
// Terry:         Boots
// Terry:         Blue Moon
// Charlotte:     Whiskers
// Arlene:

now you are able to include elements from the left even if that element has no matches in the right, in our case we retrived Arlene even he has no matching in the right

here is the reference

How to: Perform Left Outer Joins (C# Programming Guide)

---

This is the general form (as already provided in other answers)

var c =
    from a in alpha
    join b in beta on b.field1 equals a.field1 into b_temp
    from b_value in b_temp.DefaultIfEmpty()
    select new { Alpha = a, Beta = b_value };

However here's an explanation that I hope will clarify what this actually means!

join b in beta on b.field1 equals a.field1 into b_temp

essentially creates a separate result set b_temp that effectively includes null 'rows' for entries on the right hand side (entries in 'b').

Then the next line:

from b_value in b_temp.DefaultIfEmpty()

..iterates over that result set, setting the default null value for the 'row' on the right hand side, and setting the result of the right hand side row join to the value of 'b_value' (i.e. the value that's on the right hand side,if there's a matching record, or 'null' if there isn't).

Now, if the right hand side is the result of a separate LINQ query, it will consist of anonymous types, which can only either be 'something' or 'null'. If it's an enumerable however (e.g. a List - where MyObjectB is a class with 2 fields), then it's possible to be specific about what default 'null' values are used for its properties:

var c =
    from a in alpha
    join b in beta on b.field1 equals a.field1 into b_temp
    from b_value in b_temp.DefaultIfEmpty( new MyObjectB { Field1 = String.Empty, Field2 = (DateTime?) null })
    select new { Alpha = a, Beta_field1 = b_value.Field1, Beta_field2 = b_value.Field2 };

This ensures that 'b' itself isn't null (but its properties can be null, using the default null values that you've specified), and this allows you to check properties of b_value without getting a null reference exception for b_value. Note that for a nullable DateTime, a type of (DateTime?) i.e. 'nullable DateTime' must be specified as the 'Type' of the null in the specification for the 'DefaultIfEmpty' (this will also apply to types that are not 'natively' nullable e.g double, float).

You can perform multiple left outer joins by simply chaining the above syntax.

---

Here's an example if you need to join more than 2 tables:

from d in context.dc_tpatient_bookingd
join bookingm in context.dc_tpatient_bookingm 
     on d.bookingid equals bookingm.bookingid into bookingmGroup
from m in bookingmGroup.DefaultIfEmpty()
join patient in dc_tpatient
     on m.prid equals patient.prid into patientGroup
from p in patientGroup.DefaultIfEmpty()

Ref: https://stackoverflow.com/a/17142392/2343

---

Here is a fairly easy to understand version using method syntax:

IEnumerable<JoinPair> outerLeft =
    lefts.SelectMany(l => 
        rights.Where(r => l.Key == r.Key)
              .DefaultIfEmpty(new Item())
              .Select(r => new JoinPair { LeftId = l.Id, RightId = r.Id }));

---

Extension method that works like left join with Join syntax

public static class LinQExtensions
{
    public static IEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
        this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, 
        Func<TOuter, TKey> outerKeySelector, 
        Func<TInner, TKey> innerKeySelector, 
        Func<TOuter, TInner, TResult> resultSelector)
    {
        return outer.GroupJoin(
            inner, 
            outerKeySelector, 
            innerKeySelector,
            (outerElement, innerElements) => resultSelector(outerElement, innerElements.FirstOrDefault()));
    }
}

just wrote it in .NET core and it seems to be working as expected.

Small test:

        var Ids = new List<int> { 1, 2, 3, 4};
        var items = new List<Tuple<int, string>>
        {
            new Tuple<int, string>(1,"a"),
            new Tuple<int, string>(2,"b"),
            new Tuple<int, string>(4,"d"),
            new Tuple<int, string>(5,"e"),
        };

        var result = Ids.LeftJoin(
            items,
            id => id,
            item => item.Item1,
            (id, item) => item ?? new Tuple<int, string>(id, "not found"));

        result.ToList()
        Count = 4
        [0]: {(1, a)}
        [1]: {(2, b)}
        [2]: {(3, not found)}
        [3]: {(4, d)}

---

I would like to add that if you get the MoreLinq extension there is now support for both homogenous and heterogeneous left joins now

http://morelinq.github.io/2.8/ref/api/html/Overload_MoreLinq_MoreEnumerable_LeftJoin.htm

example:

//Pretend a ClientCompany object and an Employee object both have a ClientCompanyID key on them

return DataContext.ClientCompany
    .LeftJoin(DataContext.Employees,                         //Table being joined
        company => company.ClientCompanyID,                  //First key
        employee => employee.ClientCompanyID,                //Second Key
        company => new {company, employee = (Employee)null}, //Result selector when there isn't a match
        (company, employee) => new { company, employee });   //Result selector when there is a match

EDIT:

In retrospect this may work, but it converts the IQueryable to an IEnumerable as morelinq does not convert the query to SQL.

You can instead use a GroupJoin as described here: https://stackoverflow.com/a/24273804/4251433

This will ensure that it stays as an IQueryable in case you need to do further logical operations on it later.

---

There are three tables: persons, schools and persons_schools, which connects persons to the schools they study in. A reference to the person with id=6 is absent in the table persons_schools. However the person with id=6 is presented in the result lef-joined grid.

List<Person> persons = new List<Person>
{
    new Person { id = 1, name = "Alex", phone = "4235234" },
    new Person { id = 2, name = "Bob", phone = "0014352" },
    new Person { id = 3, name = "Sam", phone = "1345" },
    new Person { id = 4, name = "Den", phone = "3453452" },
    new Person { id = 5, name = "Alen", phone = "0353012" },
    new Person { id = 6, name = "Simon", phone = "0353012" }
};

List<School> schools = new List<School>
{
    new School { id = 1, name = "Saint. John's school"},
    new School { id = 2, name = "Public School 200"},
    new School { id = 3, name = "Public School 203"}
};

List<PersonSchool> persons_schools = new List<PersonSchool>
{
    new PersonSchool{id_person = 1, id_school = 1},
    new PersonSchool{id_person = 2, id_school = 2},
    new PersonSchool{id_person = 3, id_school = 3},
    new PersonSchool{id_person = 4, id_school = 1},
    new PersonSchool{id_person = 5, id_school = 2}
    //a relation to the person with id=6 is absent
};

var query = from person in persons
            join person_school in persons_schools on person.id equals person_school.id_person
            into persons_schools_joined
            from person_school_joined in persons_schools_joined.DefaultIfEmpty()
            from school in schools.Where(var_school => person_school_joined == null ? false : var_school.id == person_school_joined.id_school).DefaultIfEmpty()
            select new { Person = person.name, School = school == null ? String.Empty : school.name };

foreach (var elem in query)
{
    System.Console.WriteLine("{0},{1}", elem.Person, elem.School);
}

---

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

---

This is a SQL syntax compare to LINQ syntax for inner and left outer joins. Left Outer Join:

http://www.ozkary.com/2011/07/linq-to-entity-inner-and-left-joins.html

"The following example does a group join between product and category. This is essentially the left join. The into expression returns data even if the category table is empty. To access the properties of the category table, we must now select from the enumerable result by adding the from cl in catList.DefaultIfEmpty() statement.

---

As per my answer to a similar question, here:

Linq to SQL left outer join using Lambda syntax and joining on 2 columns (composite join key)

Get the code here, or clone my github repo, and play!

Query:

        var petOwners =
            from person in People
            join pet in Pets
            on new
            {
                person.Id,
                person.Age,
            }
            equals new
            {
                pet.Id,
                Age = pet.Age * 2, // owner is twice age of pet
            }
            into pets
            from pet in pets.DefaultIfEmpty()
            select new PetOwner
            {
                Person = person,
                Pet = pet,
            };

Lambda:

        var petOwners = People.GroupJoin(
            Pets,
            person => new { person.Id, person.Age },
            pet => new { pet.Id, Age = pet.Age * 2 },
            (person, pet) => new
            {
                Person = person,
                Pets = pet,
            }).SelectMany(
            pet => pet.Pets.DefaultIfEmpty(),
            (people, pet) => new
            {
                people.Person,
                Pet = pet,
            });

---

Perform left outer joins in linq C# // Perform left outer joins

class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

class Child
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}
public class JoinTest
{
    public static void LeftOuterJoinExample()
    {
        Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
        Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
        Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
        Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };

        Child barley = new Child { Name = "Barley", Owner = terry };
        Child boots = new Child { Name = "Boots", Owner = terry };
        Child whiskers = new Child { Name = "Whiskers", Owner = charlotte };
        Child bluemoon = new Child { Name = "Blue Moon", Owner = terry };
        Child daisy = new Child { Name = "Daisy", Owner = magnus };

        // Create two lists.
        List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
        List<Child> childs = new List<Child> { barley, boots, whiskers, bluemoon, daisy };

        var query = from person in people
                    join child in childs
                    on person equals child.Owner into gj
                    from subpet in gj.DefaultIfEmpty()
                    select new
                    {
                        person.FirstName,
                        ChildName = subpet!=null? subpet.Name:"No Child"
                    };
                       // PetName = subpet?.Name ?? String.Empty };

        foreach (var v in query)
        {
            Console.WriteLine($"{v.FirstName + ":",-25}{v.ChildName}");
        }
    }

    // This code produces the following output:
    //
    // Magnus:        Daisy
    // Terry:         Barley
    // Terry:         Boots
    // Terry:         Blue Moon
    // Charlotte:     Whiskers
    // Arlene:        No Child

https://dotnetwithhamid.blogspot.in/

---

Here's a version of the extension method solution using IQueryable instead of IEnumerable

public class OuterJoinResult<TLeft, TRight>
{
    public TLeft LeftValue { get; set; }
    public TRight RightValue { get; set; }
}

public static IQueryable<TResult> LeftOuterJoin<TLeft, TRight, TKey, TResult>(this IQueryable<TLeft> left, IQueryable<TRight> right, Expression<Func<TLeft, TKey>> leftKey, Expression<Func<TRight, TKey>> rightKey, Expression<Func<OuterJoinResult<TLeft, TRight>, TResult>> result)
{
    return left.GroupJoin(right, leftKey, rightKey, (l, r) => new { l, r })
          .SelectMany(o => o.r.DefaultIfEmpty(), (l, r) => new OuterJoinResult<TLeft, TRight> { LeftValue = l.l, RightValue = r })
          .Select(result);
}

---

If you need to join and filter on something, that can be done outside of the join. Filter can be done after creating the collection.

In this case if I do this in the join condition I reduce the rows that are returned.

Ternary condition is used (= n == null ? "__" : n.MonDayNote,)

If the object is null (so no match), then return what is after the ?. __, in this case.

Else, return what is after the :, n.MonDayNote.

Thanks to the other contributors that is where I started with my own issue.

        var schedLocations = (from f in db.RAMS_REVENUE_LOCATIONS
              join n in db.RAMS_LOCATION_PLANNED_MANNING on f.revenueCenterID equals

                  n.revenueCenterID into lm

              from n in lm.DefaultIfEmpty()

              join r in db.RAMS_LOCATION_SCHED_NOTE on f.revenueCenterID equals r.revenueCenterID
              into locnotes

              from r in locnotes.DefaultIfEmpty()
              where f.LocID == nLocID && f.In_Use == true && f.revenueCenterID > 1000

              orderby f.Areano ascending, f.Locname ascending
              select new
              {
                  Facname = f.Locname,
                  f.Areano,
                  f.revenueCenterID,
                  f.Locabbrev,

                  //  MonNote = n == null ? "__" : n.MonDayNote,
                  MonNote = n == null ? "__" : n.MonDayNote,
                  TueNote = n == null ? "__" : n.TueDayNote,
                  WedNote = n == null ? "__" : n.WedDayNote,
                  ThuNote = n == null ? "__" : n.ThuDayNote,

                  FriNote = n == null ? "__" : n.FriDayNote,
                  SatNote = n == null ? "__" : n.SatDayNote,
                  SunNote = n == null ? "__" : n.SunDayNote,
                  MonEmpNbr = n == null ? 0 : n.MonEmpNbr,
                  TueEmpNbr = n == null ? 0 : n.TueEmpNbr,
                  WedEmpNbr = n == null ? 0 : n.WedEmpNbr,
                  ThuEmpNbr = n == null ? 0 : n.ThuEmpNbr,
                  FriEmpNbr = n == null ? 0 : n.FriEmpNbr,
                  SatEmpNbr = n == null ? 0 : n.SatEmpNbr,
                  SunEmpNbr = n == null ? 0 : n.SunEmpNbr,
                  SchedMondayDate = n == null ? dMon : n.MondaySchedDate,
                  LocNotes = r == null ? "Notes: N/A" : r.LocationNote

              }).ToList();
                Func<int, string> LambdaManning = (x) => { return x == 0 ? "" : "Manning:" + x.ToString(); };
        DataTable dt_ScheduleMaster = PsuedoSchedule.Tables["ScheduleMasterWithNotes"];
        var schedLocations2 = schedLocations.Where(x => x.SchedMondayDate == dMon);

---

class Program
{
    List<Employee> listOfEmp = new List<Employee>();
    List<Department> listOfDepart = new List<Department>();

    public Program()
    {
        listOfDepart = new List<Department>(){
            new Department { Id = 1, DeptName = "DEV" },
            new Department { Id = 2, DeptName = "QA" },
            new Department { Id = 3, DeptName = "BUILD" },
            new Department { Id = 4, DeptName = "SIT" }
        };


        listOfEmp = new List<Employee>(){
            new Employee { Empid = 1, Name = "Manikandan",DepartmentId=1 },
            new Employee { Empid = 2, Name = "Manoj" ,DepartmentId=1},
            new Employee { Empid = 3, Name = "Yokesh" ,DepartmentId=0},
            new Employee { Empid = 3, Name = "Purusotham",DepartmentId=0}
        };

    }
    static void Main(string[] args)
    {
        Program ob = new Program();
        ob.LeftJoin();
        Console.ReadLine();
    }

    private void LeftJoin()
    {
        listOfEmp.GroupJoin(listOfDepart.DefaultIfEmpty(), x => x.DepartmentId, y => y.Id, (x, y) => new { EmpId = x.Empid, EmpName = x.Name, Dpt = y.FirstOrDefault() != null ? y.FirstOrDefault().DeptName : null }).ToList().ForEach
            (z =>
            {
                Console.WriteLine("Empid:{0} EmpName:{1} Dept:{2}", z.EmpId, z.EmpName, z.Dpt);
            });
    }
}

class Employee
{
    public int Empid { get; set; }
    public string Name { get; set; }
    public int DepartmentId { get; set; }
}

class Department
{
    public int Id { get; set; }
    public string DeptName { get; set; }
}

OUTPUT

---

Overview: In this code snippet, I demonstrate how to group by ID where Table1 and Table2 have a one to many relationship. I group on Id, Field1, and Field2. The subquery is helpful, if a third Table lookup is required and it would have required a left join relationship. I show a left join grouping and a subquery linq. The results are equivalent.

class MyView
{
public integer Id {get,set};
    public String Field1  {get;set;}
public String Field2 {get;set;}
    public String SubQueryName {get;set;}                           
}

IList<MyView> list = await (from ci in _dbContext.Table1
                                               join cii in _dbContext.Table2
                                                   on ci.Id equals cii.Id

                                               where ci.Field1 == criterion
                                               group new
                                               {
                                                   ci.Id
                                               } by new { ci.Id, cii.Field1, ci.Field2}

                                           into pg
                                               select new MyView
                                               {
                                                   Id = pg.Key.Id,
                                                   Field1 = pg.Key.Field1,
                                                   Field2 = pg.Key.Field2,
                                                   SubQueryName=
                                                   (from chv in _dbContext.Table3 where chv.Id==pg.Key.Id select chv.Field1).FirstOrDefault()
                                               }).ToListAsync<MyView>();


 Compared to using a Left Join and Group new

IList<MyView> list = await (from ci in _dbContext.Table1
                                               join cii in _dbContext.Table2
                                                   on ci.Id equals cii.Id

                       join chv in _dbContext.Table3
                                                  on cii.Id equals chv.Id into lf_chv
                                                from chv in lf_chv.DefaultIfEmpty()

                                               where ci.Field1 == criterion
                                               group new
                                               {
                                                   ci.Id
                                               } by new { ci.Id, cii.Field1, ci.Field2, chv.FieldValue}

                                           into pg
                                               select new MyView
                                               {
                                                   Id = pg.Key.Id,
                                                   Field1 = pg.Key.Field1,
                                                   Field2 = pg.Key.Field2,
                                                   SubQueryName=pg.Key.FieldValue
                                               }).ToListAsync<MyView>();

---

(from a in db.Assignments
     join b in db.Deliveryboys on a.AssignTo equals b.EmployeeId  

     //from d in eGroup.DefaultIfEmpty()
     join  c in  db.Deliveryboys on a.DeliverTo equals c.EmployeeId into eGroup2
     from e in eGroup2.DefaultIfEmpty()
     where (a.Collected == false)
     select new
     {
         OrderId = a.OrderId,
         DeliveryBoyID = a.AssignTo,
         AssignedBoyName = b.Name,
         Assigndate = a.Assigndate,
         Collected = a.Collected,
         CollectedDate = a.CollectedDate,
         CollectionBagNo = a.CollectionBagNo,
         DeliverTo = e == null ? "Null" : e.Name,
         DeliverDate = a.DeliverDate,
         DeliverBagNo = a.DeliverBagNo,
         Delivered = a.Delivered

     });

---