Need Help with a Trigger for preventing a circular reference on a table

We currently have a table that looks like this:

ChildId    |  ParentID
------------------
1          | NULL
2          | NULL
3          | 1
4          | 3

Can anyone help me come up with a constraint/trigger that would开发者_运维问答 prevent a circular reference from being inserted or updated into the table?

Thanks

You will need to create a trigger.