LINQ中的LEFT OUTER JOIN-Java 学习之路

404

如何在不使用 join-on-equals-into 子句的情况下在C＃LINQ中对对象执行左外连接？用 where 子句有没有办法做到这一点？正确的问题：对于内连接很容易，我有这样的解决方案

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})

但对于左外连接我需要一个解决方案 . 我是这样的，但它不起作用

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
                                        })

JoinPair是一个类：

public class JoinPair { long leftId; long rightId; }

19 回答

1

这是与LINQ比较的SQL语法内外连接的语法 . 左外连接：

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

“以下示例在产品和类别之间进行组连接 . 这实际上是左连接 . 即使类别表为空，into表达式也会返回数据 . 要访问类别表的属性，我们现在必须从可枚举的结果中进行选择通过在catList.DefaultIfEmpty（）语句中添加from cl .

回复于 2024-05-07T03:10:23+08:00
7
LEFT OUTER JOIN 的简单解决方案：
```
var setA = context.SetA;
var setB = context.SetB.Select(st=>st.Id).Distinct().ToList();
var leftOuter  = setA.Where(stA=> !setB.Contains(stA.Id));
```
notes ：
- 要提高性能，可以将SetB转换为 Dictionary （如果已完成，则必须更改此项： !setB.Contains(stA.Id) ）或 HashSet
- 当涉及多个字段时，可以使用 Set 操作和实现的类来实现： IEqualityComparer
回复于 2024-05-07T03:10:23+08:00

486

看看这个example . 此查询应该有效：

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 };

回复于 2024-05-07T03:10:23+08:00

有三个表格：人员，学校和人员学校，它们将人们与他们所在的学校联系起来 . 在表人员学校中没有提到id = 6的人 . 但是，id = 6的人在结果lef-joined网格中显示 .

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);
}

回复于 2024-05-07T03:10:23+08:00

使用lambda表达式

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   
  })

回复于 2024-05-07T03:10:23+08:00

通过扩展方法实现左外连接可能看起来像

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resultselector必须处理null元素 . 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));
   }

回复于 2024-05-07T03:10:23+08:00

Necromancing .
如果使用数据库驱动的LINQ提供程序，则可以如下编写更易读的左外连接：

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

如果省略 DefaultIfEmpty() ，则会有内连接 .

接受接受的答案：

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

这种语法非常混乱，当你想要离开连接MULTIPLE表时，它是如何工作的还不清楚 .

Note
应该注意的是 from alias in Repo.whatever.Where(condition).DefaultIfEmpty() 与外部应用/左连接 - 横向相同，任何（体面的）数据库优化器完全能够转换为左连接，只要你不在Linq中执行此操作即可 . -2对象（因为在使用Linq-to-Objects时没有DB优化器） .

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();

当与LINQ 2 SQL一起使用时，它将很好地转换为以下非常清晰的SQL查询：

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:

有关更复杂的示例，另请参阅“Convert SQL Server query to Linq query” .

另外，如果你在Linq-2-Objects（而不是Linq-2-SQL）中进行它，你应该采用老式的方式（因为LINQ to SQL正确地将它转换为连接操作，但是对于这个方法的对象）强制完全扫描，并没有利用索引搜索，为什么...）：

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
    }

);

回复于 2024-05-07T03:10:23+08:00

0
这是一般形式（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 };
```
不过这里有一个解释，我希望能澄清这实际意味着什么！
```
join b in beta on b.field1 equals a.field1 into b_temp
```
实际上创建了一个单独的结果集b_temp，它有效地包含右侧条目的空“行”（“b”中的条目） .

然后是下一行：
```
from b_value in b_temp.DefaultIfEmpty()
```
..对结果集进行设置，在右侧设置'row'的默认空值，并将右侧行连接的结果设置为'b_value'的值（即右边的值）手边，如果有匹配的记录，或者'null'，如果没有） .

现在，如果右侧是单独的LINQ查询的结果，它将由匿名类型组成，它们只能是“某事”或“空” . 如果它是一个可枚举的（例如一个List - 其中MyObjectB是一个有2个字段的类），那么就可以具体说明默认的'null'值用于它的属性：
```
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 };
```
这确保'b'本身不为null（但是它的属性可以为null，使用您指定的默认空值），这允许您检查b_value的属性而不会获得b_value的空引用异常 . 请注意，对于可为空的DateTime，必须在“DefaultIfEmpty”的规范中将（DateTime？）的类型（即“可空的DateTime”）指定为null的“Type”（这也适用于非本机的类型） '可空，例如双，漂浮） .

您只需链接上述语法即可执行多个左外连接 .
回复于 2024-05-07T03:10:23+08:00

在linq中执行左外连接C＃//执行左外连接

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/

回复于 2024-05-07T03:10:23+08:00

如果您需要加入并过滤某些内容，可以在联接之外完成 . 可以在创建集合后完成过滤 .

在这种情况下，如果我在连接条件中执行此操作，我会减少返回的行 .

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

如果对象是 null （因此不匹配），则返回 ? 之后的内容 . __ ，在这种情况下 .
否则，返回 : 之后的内容， n.MonDayNote .

感谢其他贡献者，我从自己的问题开始 .

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);

回复于 2024-05-07T03:10:23+08:00

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:

现在你能够 include elements from the left 即使那个元素 has no matches in the right ，在我们的例子中我们已经重新获得 Arlene ，即使他在右边没有匹配

这是参考

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

回复于 2024-05-07T03:10:23+08:00

如上所述：

101 LINQ Samples - Left outer join

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 };

回复于 2024-05-07T03:10:23+08:00

扩展方法，类似于左连接和Join语法

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()));
    }
}

刚刚在.NET核心中编写它，它似乎按预期工作 .

小测试：

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)}

回复于 2024-05-07T03:10:23+08:00

我想补充一点，如果你得到MoreLinq扩展，现在支持同源和异构左连接

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

例：

//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

编辑：

回想起来，这可能有效，但它将IQueryable转换为IEnumerable，因为morelinq不会将查询转换为SQL .

您可以使用此处所述的GroupJoin：https://stackoverflow.com/a/24273804/4251433

这将确保它保持为IQueryable，以防您以后需要对其进行进一步的逻辑操作 .

回复于 2024-05-07T03:10:23+08:00

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

回复于 2024-05-07T03:10:23+08:00

436

现在作为扩展方法：

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));
    }
}

使用就像你通常使用join一样：

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

希望这能为您节省一些时间 .

回复于 2024-05-07T03:10:23+08:00

如果您需要加入两个以上的表，这是一个示例：

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()

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

回复于 2024-05-07T03:10:23+08:00

这是一个使用方法语法的相当容易理解的版本：

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 }));

回复于 2024-05-07T03:10:23+08:00

(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

     });

回复于 2024-05-07T03:10:23+08:00

LINQ中的LEFT OUTER JOIN

19 回答

相关问题