LINQ 中的左外連接

Question

如何在不使用join-on-equals-into子句的情況下對對象執行 C# LINQ 中的左外連接？ 有沒有辦法用where子句做到這一點？ 正確的問題：對於內部連接很容易，我有這樣的解決方案

List<JoinPair> innerFinal = (from l in lefts from r in rights where l.Key == r.Key
                             select new JoinPair { LeftId = l.Id, RightId = r.Id})

但對於左外連接，我需要一個解決方案。 我的是這樣的，但它不工作

List< JoinPair> leftFinal = (from l in lefts from r in rights
                             select new JoinPair { 
                                            LeftId = l.Id, 
                                            RightId = ((l.Key==r.Key) ? r.Id : 0
                                        })

其中JoinPair是 class：

public class JoinPair { long leftId; long rightId; }

Answer 1

如“執行左外連接”中所述：

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 };

Answer 2

如果使用數據庫驅動的 LINQ 提供程序，則可以這樣編寫可讀性更高的左外連接：

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

如果您省略DefaultIfEmpty()您將有一個內部連接。

接受公認的答案：

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

這種語法非常令人困惑，當您想要左連接 MULTIPLE 表時，不清楚它是如何工作的。

筆記
應該注意的是from alias in Repo.whatever.Where(condition).DefaultIfEmpty()與外部應用/左連接橫向相同，任何（體面的）數據庫優化器都完全能夠將其轉換為左連接，只要您不引入每行值（也就是實際的外部應用）。 不要在 Linq-2-Objects 中執行此操作（因為使用 Linq-to-Objects 時沒有 DB-optimizer）。

詳細示例

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();

當與 LINQ 2 SQL 一起使用時，它將很好地轉換為以下非常清晰的 SQL 查詢：

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

編輯：

另請參閱“ 將 SQL Server 查詢轉換為 Linq 查詢”以獲得更復雜的示例。

此外，如果您在 Linq-2-Objects（而不是 Linq-2-SQL）中執行此操作，則應該以老式方式執行此操作（因為 LINQ to SQL 將其正確轉換為連接操作，但在對象上使用此方法強制進行全面掃描，並且不利用索引搜索，但是...）：

    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
    }

);

Answer 3

使用 lambda 表達式

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   
  });

Answer 4

現在作為擴展方法：

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));
    }
}

像通常使用 join 一樣使用：

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

希望這可以節省您一些時間。

Answer 5

看看這個例子。 此查詢應該有效：

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 };

Answer 6

通過擴展方法實現左外連接可能看起來像

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));
     }
   }

然后結果選擇器必須處理空元素。 外匯。

   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));
   }

Answer 7

看看這個例子

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:

現在您可以include elements from the left即使該元素has no matches in the right項，在我們的例子中，我們檢索了Arlene ，即使他在右側沒有匹配項

這是參考

如何：執行左外連接（C# 編程指南）

Answer 8

這是一般形式（已在其他答案中提供）

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 };

然而，這里有一個解釋，我希望能澄清這實際上意味着什么！

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

本質上創建了一個單獨的結果集 b_temp，它有效地包括右側條目的空“行”（“b”中的條目）。

然后是下一行：

from b_value in b_temp.DefaultIfEmpty()

..迭代該結果集，為右側的“行”設置默認空值，並將右側行連接的結果設置為“b_value”的值（即右側的值手邊，如果有匹配的記錄，如果沒有，則為“null”）。

現在，如果右側是單獨的 LINQ 查詢的結果，它將由匿名類型組成，只能是“某物”或“空”。 但是，如果它是可枚舉的（例如 List - 其中 MyObjectB 是具有 2 個字段的類），則可以具體說明其屬性使用的默認“null”值：

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 };

這確保了 'b' 本身不為空（但它的屬性可以為空，使用您指定的默認空值），這允許您檢查 b_value 的屬性，而不會獲得 b_value 的空引用異常。 請注意，對於可為空的 DateTime，（DateTime？）類型，即“可為空的 DateTime”，必須在“DefaultIfEmpty”的規范中指定為 null 的“類型”（這也適用於不是“本機”的類型' 可以為空，例如 double、float)。

您可以通過簡單地鏈接上述語法來執行多個左外連接。

Answer 9

如果您需要加入 2 個以上的表，以下是一個示例：

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()

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

Answer 10

這是一個使用方法語法的相當容易理解的版本：

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 }));

Answer 11

擴展方法，類似於使用 Join 語法的左連接

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()));
    }
}

剛剛在 .NET 核心中編寫它，它似乎按預期工作。

小測試：

        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)}

Answer 12

我想補充一點，如果您獲得了 MoreLinq 擴展，現在現在支持同質和異構左連接

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

例子：

//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

編輯：

回想起來，這可能有效，但它將 IQueryable 轉換為 IEnumerable，因為 morelinq 不會將查詢轉換為 SQL。

您可以改為使用 GroupJoin，如下所述： https ://stackoverflow.com/a/24273804/4251433

這將確保它保持為 IQueryable，以防您稍后需要對其進行進一步的邏輯操作。

Answer 13

共有三個表：persons、schools 和 persons_schools，將人員與他們就讀的學校聯系起來。persons_schools 表中沒有對 id=6 的人員的引用。 然而，id=6 的人顯示在結果左連接網格中。

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);
}

Answer 14

簡單的方法是使用 Let 關鍵字。 這對我有用。

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
}

這是Left Join的模擬。 如果 B 表中的每個項目都與 A 項目不匹配，則 BItem 返回 null

Answer 15

這是一種 SQL 語法，與用於內連接和左外連接的 LINQ 語法相比。 左外連接：

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

“下面的例子在產品和類別之間進行組連接。這本質上是左連接。即使類別表為空，into 表達式也會返回數據。要訪問類別表的屬性，我們現在必須從可枚舉的結果中進行選擇通過在 catList.DefaultIfEmpty() 語句中添加 from cl。

Answer 16

根據我對類似問題的回答，在這里：

Linq to SQL 使用 Lambda 語法左外連接並連接 2 列（復合連接鍵）

在這里獲取代碼，或者克隆我的 github 倉庫，然后玩！

詢問：

        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,
            };

拉姆達：

        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,
            });

Answer 17

在 linq C# 中執行左外連接 // 執行左外連接

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/

Answer 18

這是使用 IQueryable 而不是 IEnumerable 的擴展方法解決方案的一個版本

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);
}

Answer 19

如果您需要加入並過濾某些內容，可以在加入之外完成。 可以在創建集合后進行過濾。

在這種情況下，如果我在連接條件中執行此操作，我會減少返回的行。

使用三元條件(= n == null ? "__" : n.MonDayNote,)

• 如果對象為null （因此不匹配），則返回 ? 之后的內容? . __ ，在這種情況下。

• 否則，返回: 、 n.MonDayNote之后的內容。

感謝其他貢獻者，這是我從自己的問題開始的地方。

---

        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);

Answer 20

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; }
}

輸出

Answer 21

概述：在此代碼段中，我演示了如何在 Table1 和 Table2 具有一對多關系的情況下按 ID 進行分組。 我對 Id、Field1 和 Field2 進行分組。 如果需要第三次表查找並且它需要左連接關系，則子查詢很有幫助。 我展示了一個左連接分組和一個子查詢 linq。 結果是等價的。

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>();

Answer 22

使用 LINQ 進行左連接的規定方法是到GroupJoin后跟SelectMany 。 我更喜歡將此邏輯抽象為LeftJoin擴展方法，以使代碼更具可讀性。

這是我使用的LeftJoin實現。 此實施的兩個主要好處是：

1. 它特別適用於IQueryable s

2. 它具有與Join方法完全相同的參數和返回類型：

    var results = DbContext.Categories.LeftJoin( DbContext.Products, c => c.Id, p => p.CategoryId, (c, p) => new { Category = c, ProductName = p == null? "(No Products)": p.ProductName }).ToList(); var results = DbContext.Categories.Join( DbContext.Products, c => c.Id, p => p.CategoryId, (c, p) => new { Category = c, ProductName = p.ProductName }).ToList();

---

就是這樣：

IQueryable 擴展：

public static class QueryableExtensions
{
    public static IQueryable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(this IQueryable<TOuter> outer, IEnumerable<TInner> inner, Expression<Func<TOuter, TKey>> outerKeySelector, Expression<Func<TInner, TKey>> innerKeySelector, Expression<Func<TOuter, TInner, TResult>> resultSelector)
    {
        var query = outer
            .GroupJoin(inner, outerKeySelector, innerKeySelector, (o, i) => new { o, i })
            .SelectMany(o => o.i.DefaultIfEmpty(), (x, i) => new { x.o, i });
        return ApplySelector(query, x => x.o, x => x.i, resultSelector);
    }

    private static IQueryable<TResult> ApplySelector<TSource, TOuter, TInner, TResult>(
        IQueryable<TSource> source,
        Expression<Func<TSource, TOuter>> outerProperty,
        Expression<Func<TSource, TInner>> innerProperty,
        Expression<Func<TOuter, TInner, TResult>> resultSelector)
    {
        var p = Expression.Parameter(typeof(TSource), $"param_{Guid.NewGuid()}".Replace("-", string.Empty));
        Expression body = resultSelector?.Body
            .ReplaceParameter(resultSelector.Parameters[0], outerProperty.Body.ReplaceParameter(outerProperty.Parameters[0], p))
            .ReplaceParameter(resultSelector.Parameters[1], innerProperty.Body.ReplaceParameter(innerProperty.Parameters[0], p));
        var selector = Expression.Lambda<Func<TSource, TResult>>(body, p);
        return source.Select(selector);
    }
}

表達式擴展：

public static class ExpressionExtensions
{
    public static Expression ReplaceParameter(this Expression source, ParameterExpression toReplace, Expression newExpression)
        => new ReplaceParameterExpressionVisitor(toReplace, newExpression).Visit(source);
}

表達訪客：

public class ReplaceParameterExpressionVisitor : ExpressionVisitor
{
    public ReplaceParameterExpressionVisitor(ParameterExpression toReplace, Expression replacement)
    {
        this.ToReplace = toReplace;
        this.Replacement = replacement;
    }

    public ParameterExpression ToReplace { get; }

    public Expression Replacement { get; }

    protected override Expression VisitParameter(ParameterExpression node)
        => (node == ToReplace) ? Replacement : base.VisitParameter(node);
}

Answer 23

這是我使用的最漂亮的解決方案，試一試！

(from c in categories
      let product = products.Where(d=> d.Category == c.Category).FirstOrDefault()
    select new { Category = c, ProductName = p == null ? "(No products)" : product.ProductName };

Answer 24

(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

     });

Answer 25

LEFT OUTER JOIN 的簡單解決方案：

var setA = context.SetA;
var setB = context.SetB.Select(st=>st.Id).Distinct().ToList();
var leftOuter  = setA.Where(stA=> !setB.Contains(stA.Id)); 

筆記：

• 為了提高性能 SetB 可以轉換為字典（如果這樣做，那么你必須改變這個： !setB.Contains(stA.Id) ）或一個HashSet
• 當涉及多個字段時，這可以使用Set操作和實現的類來實現： IEqualityComparer