看一下这个例子。 这个查询应该工作:

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 };

有三个表:persons, schools和persons_schools，它将人与他们所学习的学校联系起来。表persons_schools中没有对id=6的person的引用。但是id=6的人显示在左连接的结果网格中。

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);
}

看一下这个例子。 这个查询应该工作:

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 };

概述:在这个代码片段中，我演示了如何按ID分组，其中Table1和Table2具有一对多关系。我集中在 Id, Field1和Field2。如果需要第三次查找表，并且需要左连接关系，那么子查询是有用的。 我展示了一个左连接分组和一个子查询linq。结果是等价的。

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>();

根据我对类似问题的回答，在这里:

Linq到SQL左外连接使用Lambda语法和连接2列(复合连接键)

获得代码在这里，或克隆我的github回购，并发挥!

查询:

        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,
            };

λ:

        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,
            });

简单的方法是使用Let关键字。这对我很有用。

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
}

这是一个模拟左连接。如果B表中的每一项与A表中的每一项都不匹配，BItem返回null