Compare two fields in CAML Query in sharepoint List -

i have share point list named "documents" contains (id , title , parentid)

where parentid refer same list , id auto generated share point.

i need create caml query select documents parentid equal id

maybe can create computed field in list compare like:

areidequals: [id] = [parentid]

and use:

<query>    <where>       <eq>          <fieldref name="areidequals" />          <value type="boolean">1</value>       </eq>    </where> </query> 

from here: http://social.msdn.microsoft.com/forums/sharepoint/en-us/ab303453-dfd6-46c4-888a-e15d233c8e57/compare-two-fields-in-caml-query