LINQ Left Join And Right Join

Question

I need a help,

I have two dataTable called A and B , i need all rows from A and matching row of B

Ex:

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

I need Out Put:

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

Answer 1

The example data and output you've provided does not demonstrate a left join. If it was a left join your output would look like this (notice how we have 3 results for user 1, i.e. once for each Growth record that user 1 has):

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  |

Assuming that you still require a left join; here's how you do a left join in Linq:

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
              };

If you want to do a right join, just swap the tables that you're selecting from over, like so:

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
              };

The important part of the code is the into statement, followed by the DefaultIfEmpty. This tells Linq that we want to have the default value (i.e. null) if there isn't a matching result in the other table.

Answer 2

Doctor Jones showed left outer join, but correct answer would be slightly different - because in the original question two tables linked on age field so to get result exactly as needed following code should be used.

....
//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

Easy way is to use Let keyword. This works for me.

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
}

This is a simulation of Left Join. If each item in B table not match to A item , BItem return null

Answer 4

Here's a simple example.
Model:

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; }
}

Method:

public void LeftRightJoin()
{
    // Example Projects
    List<Project> ListOfProjects = new()
    {
        new(){ ProjectID = 1, ProjectName = "UID" },
        new(){ ProjectID = 2, ProjectName = "RBS" },
        new(){ ProjectID = 3, ProjectName = "XYZ" },
    };
    // Example Employees
    List<Employee> ListOfEmployees = new(){
        new(){ ID = 1, Name = "Sunil",  ProjectID = 1 },
        new(){ ID = 1, Name = "Anil", ProjectID = 1 },
        new(){ ID = 1, Name = "Suman", ProjectID = 2 },
        new(){ ID = 1, Name = "Ajay", ProjectID = 3 },
        new(){ ID = 1, Name = "Jimmy", ProjectID = 4 }
    };

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

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

    //Printing result of left join
    Console.WriteLine(string.Join("\n", Ljoin.Select(emp => $" Employee Name = {emp.EmployeeName}, Project Name = {emp.ProjectName}")));

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