find root nodes hierarchical tree

i have list database stores tree structure. here applicable fields:

mytree (id, parentid, otherfields...)

i wish root nodes (that is, any record whose id another record's parentid)

i've attempted this:

select * mytree where `id` (select graphic `parentid` `mytree`)

but returned an lifeless set. strangely, stealing "not" advantage set non-leaf nodes.

can anyone where i'm going wrong?

update: interjection answers folks, have repremand worked me. i've supposed daniel's given also explains since query didn't work (the zero thing).