| id | text                     | parent_id |
=============================================
| 1  | Blah                     | 0         | Root Comment
| 2  | Blah                     | 0         | Root Comment
| 3  | Blah                     | 1         | Child of 1st comment
| 4  | Blah                     | 1         | Another Child...
| 5  | Blah                     | 3         | etc...

and then i delete for example comment with id 1:

| id | text                     | parent_id |
=============================================
| 2  | Blah                     | 0         | Root Comment
| 3  | Blah                     | 1         | Child of 1st comment
| 4  | Blah                     | 1         | Another Child...
| 5  | Blah                     | 3         | etc...

and i'm left with 3,4,5 orphans, i hope you understand what i mean now :)