Using linq to check if not exists clause

Question

Here is situation I have been trying to solve

Lets take a Employee table

Create Table Employee
(
        Employeeid int primary key,
        EMPname varchar(50),
        ManagerEmplId int reference key Employee (EmployeeID)
         TreeLevel int,
              ....
)

Here i need to find all leaf level employees.

Leaf Level Employees - All employees who have manager but they do not have anybody reporting to them. I have a little help from db which has TreeLevel column where I can specify pick anybody at level 3 but I need a UNIONclause which will get me all employees at treelevel 2 that do not have any employees reporting. I have only 3 levels of tree if that helps in creating linq query.

   return ((from b in _db.Employees
                && b.TreeLevel==3 && b.DeletedDate== null
                    select b)
                    .Union
                    (from b in _db.Employees

                     select b)

                    )
                    .ToDictionary(k => k.EmployeeID, v => v.EMPname);

UPDATE: The real query:

(from fi in firm 
 join bra in _db.Branches on fi.BranchID equals bra.ParentBranchID into g 
 from sc in g.DefaultIfEmpty() 
 where fi.DeletedDate == null && g == null 
 select fi)
 .ToList()
 .ToDictionary(k => k.BranchID, v => v.BranchName);

Error:

Cannot compare elements of type 'System.Collections.Generic.IEnumerable`1'. 
Only primitive types (such as Int32, String, and Guid) and entity types are supported.

Answer 1

This query should do the trick, regardless of tree depth:

var leafEmps = 
    (from emp in _db.Employees
     where !_db.Employees.Any(e => e.ManagerEmplId == emp.EmployeeId)
     select emp);

Answer 2

You can try the right outer join and make sure that the left side is empty.

In this post How to do a full outer join in Linq? you can find a good example of how to do it in linq.

from b in _db.Employees
from c in _db.Employees.Where(o=> o.ManagerEmplId == b.Id).DefaultIfEmpty()
where c == null

Answer 3

var managersids = _db.Employees.Select(emp => emp.ManagerEmplId ).Distinct();
var leafemps = _db.Employees.where(emp => !managersids.contains(emp.Employeeid));

To do this simple, get all the managers and then search the people who arn't a manager

var leafemps = from emp in _db.Employees
               let managersids = _db.Employees.Select(emp => emp.ManagerEmplId ).Distinct()
               where !managersids.contains(emp.Employeeid)
               select emp