LINQ 左連接和右連接

Question

我需要幫助，

我有兩個名為 A 和 B 的數據表，我需要 A 中的所有行和 B 的匹配行

前任：

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

我需要輸出：

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

Answer 1

您提供的示例數據和輸出並未演示左連接。 如果是左連接，您的輸出將如下所示（注意我們如何為用戶 1 獲得 3 個結果，即用戶 1 擁有的每條增長記錄一次）：

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  |

假設您仍然需要左連接； 以下是在 Linq 中執行左連接的方法：

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
              };

如果您想進行正確的連接，只需交換您從中選擇的表，如下所示：

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
              };

代碼的重要部分是 into 語句，然后是DefaultIfEmpty 。 這告訴 Linq，如果另一個表中沒有匹配結果，我們希望使用默認值（即 null）。

Answer 2

瓊斯醫生展示了左外連接，但正確答案會略有不同 - 因為在原始問題中，兩個表在年齡字段上鏈接，因此應使用以下代碼來完全根據需要獲得結果。

....
//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; }
    }

Answer 3

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.FirstOrDefault(x => x.id == AItem.id ) 
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 4

簡單示例

模型

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

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("\n", Ljoin.Select(emp => " Employee Name = " +
    emp.EmployeeName + ", Project Name = " + emp.ProjectName).ToArray<string>()));

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