Listing parent ids

Not strictly a php issue but it's for a php app, hope that counts (plus I haven't had much joy googling this) I have a table with an id and a parentid. If it's a top-level record the parentid is 0, otherwise it points to another record, and if that record isn't a top-level record its parentid points to another record, etc (a linked list). Is there a single select that will return the complete list of parentids? Or do I have to iterate selecting each parent record while parentid > 0 and build the list entry by entry? TIA Cheers, Arno