hierarchical XML in a relational DB structure - moving nodes

[Robert Koberg <rob@koberg.com>]

Hi,

I want to use a relational database to store data that is nested and 
where the order is preserved. I am having a hard time wrapping my head 
around how to do the following mostly because I don't have much 
experience with relational databases.

I am looking at mapping a structure like so:

<node id="1">
   <node id="11">
     <node id="111">
       <node id="1111"/>
       <node id="1112"/>
       <node id="1113"/>
     </node>
     <node id="112"/>
     <node id="113"/>
     <node id="114"/>
   </node>
   <node id="12">
     <node id="121"/>
     <node id="122"/>
     <node id="123"/>
   </node>
</node>

To a relational database that might have a table representing it like:

Node
-------
id
parentId
parentIndexPosition

First question: Is using an index position the best way to maintain the 
position? (or is preceding and/or following sibling better? or something 
else?)

OK, lets say I have imported some data/XML into the above table, giving 
appropriate parents and index positions.

Next, say I want to move /node/node[2]/node[@id=121] to the position 
/node/node[1]/node[1].

How do you handle updating the index position of other nodes? All 
children of /node/node[@id=12] will need their parentIndexPosition moved 
down one. And all children of /node/node[@id=11] will need their 
parentIndexPosition moved up one. (the ID attributes where abritrarily 
created and should not indicate position)

Is there a built in mechanism to do this type of thing in relational 
databases? If not how would an UPDATE look?

Are there any java libraries or articles out there that handle this type 
of thing? (I am having a hard time finding anything with google)

thanks,
-Rob