如何在c# LINQ中执行左外连接到对象而不使用join-on-equal -into子句?有办法用where子句来实现吗? 正确的问题: 内连接很简单,我有一个这样的解决方案

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})

但是对于左外连接,我需要一个解决方案。我的是这样的,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是一个类:

public class JoinPair { long leftId; long rightId; }

当前回答

这是一般形式(已经在其他答案中提供了)

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,这里有一个解释,我希望能澄清这实际上是什么意思!

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

本质上创建了一个单独的结果集b_temp,其中有效地包含了右边条目的空“rows”('b'中的条目)。

然后是下一行:

from b_value in b_temp.DefaultIfEmpty()

..遍历该结果集,为右侧的'row'设置默认空值,并将右侧的行连接的结果设置为'b_value'的值(即,如果有匹配的记录,则为右侧的值,如果没有则为'null')。

现在,如果右边是一个单独的LINQ查询的结果,它将由匿名类型组成,它只能是'something'或'null'。如果它是一个可枚举的对象(例如,一个List -其中MyObjectB是一个有2个字段的类),那么它可以具体说明它的属性使用什么默认的“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 };

这确保了'b'本身不是空的(但它的属性可以是空的,使用您指定的默认空值),这允许您检查b_value的属性,而不会得到b_value的空引用异常。注意,对于可空的DateTime, (DateTime?)类型,即。'nullable DateTime'必须在'DefaultIfEmpty'的规范中指定为null的'Type'(这也适用于'原生'不为空的类型,例如double, float)。

您可以通过简单地链接上述语法来执行多个左外连接。

其他回答

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 };

下面是一个例子,如果你需要连接2个以上的表:

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()

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

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; }
}

输出

(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

     });

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 };