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