看看这个例子

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:

现在你可以从左边包含元素，即使那个元素在右边没有匹配，在我们的例子中，我们检索了Arlene，即使他在右边没有匹配

这是参考资料

如何:执行左外连接(c#编程指南)

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));
   }

如“Perform left outer joins”中所述:

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

如果需要连接和筛选某些东西，可以在连接之外完成。可以在创建集合之后进行筛选。

在这种情况下，如果我在连接条件中这样做，我减少了返回的行。

使用三元条件(= n == null ?"__": n.MonDayNote，)

如果对象为空(因此不匹配)，则返回?后面的内容。__，在这种情况下。 否则，返回:，n.MonDayNote后面的内容。

感谢其他贡献者，这是我开始自己的问题。

        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);

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

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);
}