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>();

(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

     });

下面是一个版本的扩展方法解决方案，使用IQueryable代替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);
}

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);
}

通过扩展方法的左外连接的实现如下所示

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));
     }
   }

然后，resultselector必须处理空元素。外汇。

   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));
   }