LINQ Left Outer Join

In LINQ, LEFT JOIN or LEFT OUTER JOIN is used to return all the records or elements from the left side collection and matching the elements from the right side of the collection.

In LINQ, to achieve the LEFT Join behavior, it is mandatory to use the "INTO" keyword and "DefaultfEmpty()" method.

Syntax of LINQ Left Outer Join

The syntax of using the LINQ Left Outer Join to get all the elements from the collection and matching the element from the right collection.

var result = from e in objEmp1
join d in objDept1
on e.DeptId equals d.DepId into empDept
from ed in empDept.DefaultIfEmpty()
select new
{
    EmployeeName = e.Name,
    DepartmentName = ed == null ? "No Department" : ed.DepName
}

From the above syntax, we used into and DefaultfEmpty() methods to implement the left outer join to get the elements from the "objEmp1", "objDept1" collections.

Example of LINQ Left Outer Join

Here is the example of using the LINQ Left Outer Join to get the elements from the collections based on the specified conditions.

using System;
using System. Collections;
using System.Collections.Generic;
using System. Linq;
using System. Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Programme2
    {
        static void Main(string[] args)
        {
/*create an object of the class 'Department'
 and create a list with the added record*/
        List<Department> objDept1 = new List<Department>()
        {
        new Department{DepId=1,DepName="Software"},
        new Department{DepId=2,DepName="Finance"},
        new Department{DepId=3,DepName="Health"}
        };
    /*create an object of the class 'Department'
    and create a list with the added record*/
           List<Employee>objEmp1 = new List<Employee>()
            {
            new Employee { EmpId=1,Name = "Akshay Tyagi", DeptId=1 },
            new Employee { EmpId=2,Name = "Vishi Tyagi", DeptId=1 },
            new Employee { EmpId=3,Name = "Arpita Rai", DeptId=2 },
            new Employee { EmpId=4,Name = "Mani", DeptId =2},
            new Employee { EmpId=5,Name = "Madhav Sai"}
            };
    /*use Linq Query to fetch the information by using the join clause 
    and check the department containing the employee or not */
            var result = from e in objEmp1
            join d in objDept1
            on e.DeptId equals d.DepId into empDept
            from ed in empDept.DefaultIfEmpty()
            select new
            {
            EmployeeName = e.Name,
            DepartmentName = ed == null ? "No Department" : ed.DepName
            };
            foreach (var item in result)
            {
            Console.WriteLine(item.EmployeeName + "\t | " + item.DepartmentName);
            }
            Console.ReadLine();
            }
            }
            class Department
            {
            public int DepId { get; set; }
            public string DepName { get; set; }
            }
            class Employee
            {
            public int EmpId { get; set; }
            public string Name { get; set; }
            public int DeptId { get; set; }
            }
            }

From the above example, we are getting the elements from "objEmp1", "objDept1" collections by using left outer join in LINQ and here we specified the condition to check if there is a department for the employee or not. In case if the department not mapped, it will take "No Department" from the condition.

OUTPUT: