I am using laravel eloquent to get the query results. I have two tables below:

users table:

```
|  id | department_id
|  1  |   1
|  2  |   3
|  3  |   2

```

department table:

```
| id  |  name
| 1   | A
| 2   | B
| 3   | C
| 4   | D
| 5   | E

```

How to get one unassigned ID, not existing department ID, into the users table? Example, 4 & 5 are not yet existing in users table, so how can I get 4 or 5 using an eloquent?

I am thinking of this but this is not correct.

```
Department::select('department.id as id')
             ->leftJoin('users', 'users.department_id' ,'department.id')
             ->pluck('id');

```

Does anybody know?