在linq中左连接中进入关键字的目的？

Question

尽管这看起来很基础，但是我在理解这个概念时仍在挣扎，左联接实际上是如何工作的？当没有分组的机会时，成关键字的目的是什么意思，然后为什么使用它来实现左联接??我们在这里如何分组？

public class LinqJoinController : Controller
    {
        //
        // GET: /LinqJoin/

        public List<Department> getAllDepartment()
        {
            return new List<Department>(){
                new Department{Id=1,Name="IT"},
                 new Department{Id=2,Name="HR"},

            };
        }

        public List<Employee> getAllEmployee()
        {
            return new List<Employee>(){
                new Employee{Id=1,Name="Sachin",DepartmentId=1},
                 new Employee{Id=2,Name="Ram",DepartmentId=1},
                  new Employee{Id=3,Name="Shyam",DepartmentId=2},
                 new Employee{Id=4,Name="Radhe"},
            };
        }
        public ActionResult Index()
        {
            var result = from  emp in getAllEmployee()
                         join dep in getAllDepartment()

                             on emp.DepartmentId equals dep.Id 
                             into eGroup
                             from dep in eGroup.DefaultIfEmpty()
                         select new EmployeeVM
                         {
                             DepartmentName = dep==null?"No Dept":dep.Name,
                             EmployeeName = emp.Name
                         };
            return View(result);
        }
    }
}