LINQ左连接和右连接

LINQ Left Join And Right Join

我需要帮助，

我有两个名为a和b的数据表，我需要a的所有行和b的匹配行

前任：

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A: B:

User | age| Data ID | age|Growth
1 |2 |43.5 1 |2 |46.5
2 |3 |44.5 1 |5 |49.5
3 |4 |45.6 1 |6 |48.5

我需要输出：

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User | age| Data |Growth
------------------------
1 |2 |43.5 |46.5
2 |3 |44.5 |
3 |4 |45.6 |

您提供的示例数据和输出并不演示左联接。如果是左连接，您的输出将如下所示(请注意，对于用户1，我们有3个结果，即，对于用户1拥有的每个增长记录，我们有一个结果)：

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User | age| Data |Growth
------------------------
1 |2 |43.5 |46.5
1 |2 |43.5 |49.5
1 |2 |43.5 |48.5
2 |3 |44.5 |
3 |4 |45.6 |

假设您仍然需要一个左联接；下面介绍如何在LINQ中执行左联接：

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var results = from data in userData
join growth in userGrowth
on data.User equals growth.User into joined
from j in joined.DefaultIfEmpty()
select new
{
UserData = data,
UserGrowth = j
};

如果要进行正确的联接，只需交换要从上方选择的表，如下所示：

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var results = from growth in userGrowth
join data in userData
on growth.User equals data.User into joined
from j in joined.DefaultIfEmpty()
select new
{
UserData = j,
UserGrowth = growth
};

代码的重要部分是into语句，后面是defaultifempty。这告诉Linq，如果另一个表中没有匹配的结果，我们希望有默认值(即空值)。

琼斯医生显示左外联接，但正确的答案会略有不同-因为在最初的问题中，两个表在年龄字段上链接，以便根据需要获得准确的结果，应使用以下代码。

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....
//ctx = dataContext class - not shown here.
var user1 = new UserData() { User = 1, Age = 2, Data = 43.5 };
var user2 = new UserData() { User = 2, Age = 3, Data = 44.5 };
var user3 = new UserData() { User = 3, Age = 4, Data = 45.6 };

ctx.UserData.AddRange(new List<UserData> { user1, user2, user3 });

var growth1 = new UserGrowth() { Id = 1, Age = 2, Growth = 46.5 };
var growth2 = new UserGrowth() { Id = 1, Age = 5, Growth = 49.5 };
var growth3 = new UserGrowth() { Id = 1, Age = 6, Growth = 48.5 };

ctx.UserGrowth.AddRange(new List<UserGrowth> { growth1, growth2, growth3 });

var query = from userData in ctx.UserData
join userGrowth in ctx.UserGrowth on userData.Age equals userGrowth.Age
into joinGroup
from gr in joinGroup.DefaultIfEmpty()
select new
{
User = userData.User,
age = userData.Age,
Data = (double?)userData.Data,
Growth = (double?)gr.Growth
};

Console.WriteLine("{0} | {1} | {2} | {3}","User","age","Data","Growth");
foreach (var x in query)
{
Console.WriteLine("{0} | {1} | {2} | {3}", x.User, x.age, x.Data, x.Growth);
}

.... with following entity classes:

public class UserData
{
[Key]
public int User { get; set; }
public int Age { get; set; }
public double Data { get; set; }
}

public class UserGrowth
{
public int Id { get; set; }
public int Age { get; set; }
public double Growth { get; set; }
}

简单实例

模型

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class Employee
{
public string Name { get; set; }
public int ID { get; set; }
public int ProjectID { get; set; }
}

class Project
{
public int ProjectID { get; set; }
public string ProjectName { get; set; }
}

方法

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public void leftrighjoin(){
Project P1 = new Project() { ProjectID = 1, ProjectName ="UID" };
Project P2 = new Project() { ProjectID = 2, ProjectName ="RBS" };
Project P3 = new Project() { ProjectID = 3, ProjectName ="XYZ" };
// Employee List
List<Employee> ListOfEmployees = new List<Employee>();
ListOfEmployees.AddRange((new Employee[]
{
new Employee() { ID = 1, Name ="Sunil", ProjectID = 1 },
new Employee() { ID = 1, Name ="Anil", ProjectID = 1 },
new Employee() { ID = 1, Name ="Suman", ProjectID = 2 },
new Employee() { ID = 1, Name ="Ajay", ProjectID = 3 },
new Employee() { ID = 1, Name ="Jimmy", ProjectID = 4 }}));

//Project List
List<Project> ListOfProject = new List<Project>();
ListOfProject.AddRange(new Project[] { P1, P2, P3 });

//Left join
var Ljoin = from emp in ListOfEmployees
join proj in ListOfProject
on emp.ProjectID equals proj.ProjectID into JoinedEmpDept
from proj in JoinedEmpDept.DefaultIfEmpty()
select new
{
EmployeeName = emp.Name,
ProjectName = proj != null ? proj.ProjectName : null
};

//Right outer join
var RJoin = from proj in ListOfProject
join employee in ListOfEmployees
on proj.ProjectID equals employee.ProjectID into joinDeptEmp
from employee in joinDeptEmp.DefaultIfEmpty()
select new
{
EmployeeName = employee != null ? employee.Name : null,
ProjectName = proj.ProjectName
};

//Printing result of left join
Console.WriteLine(string.Join("
", Ljoin.Select(emp =>" Employee Name =" +
emp.EmployeeName +", Project Name =" + emp.ProjectName).ToArray<string>()));

//printing result of right outer join
Console.WriteLine(string.Join("
", RJoin.Select(emp =>" Employee Name =" +
emp.EmployeeName +", Project Name =" + emp.ProjectName).ToArray<string>()));
}

简单的方法是使用let关键字。这对我有用。

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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
}

这是对左连接的模拟。如果B表中的每个项与某个项不匹配，则b item返回空值