SQL Question

I have a table called ‘tasks’ with a structure like this:

Taskid            ParentId

----------------------------

1                                                                    null

2                                                                    1

3                                                                    1

4                                                                    Null

5                                                                    4

6                                                                    4

7                                                                    Null

8                                                                    Null

9                                                                    15

10                                                                18

The taskid column is the primary key and the parentid column references values in the taskid column.  However, it is possible for the parentid to be a value that is not found in the taskid column (hence my question…).  I need to write a query that will retrieve all rows where the parentid is either ‘null’ or does not exist in the taskid column.

Any help would really be appreciated.

Thanks!

Kevin