Getting Hierarchial Query as XML (and maitaining the relationship)

Question

Pls find below sample table script and desired output format.XML output should maintain the hierarchy info which is the requirement.Also is it possible to do in single sql.
Thanks.
CREATE TABLE MAIN
ID NUMBER,
NAME VARCHAR2(20 BYTE)
INSERT INTO MAIN ( ID, NAME ) VALUES (
1, 'OS');
INSERT INTO MAIN ( ID, NAME ) VALUES (
2, 'Windows');
INSERT INTO MAIN ( ID, NAME ) VALUES (
3, 'DOS');
INSERT INTO MAIN ( ID, NAME ) VALUES (
4, '1.x');
INSERT INTO MAIN ( ID, NAME ) VALUES (
5, '1.1.x');
INSERT INTO MAIN ( ID, NAME ) VALUES (
7, '1.1.1.1');
INSERT INTO MAIN ( ID, NAME ) VALUES (
8, '1.2');
INSERT INTO MAIN ( ID, NAME ) VALUES (
9, '2.x');
INSERT INTO MAIN ( ID, NAME ) VALUES (
10, '2.1');
INSERT INTO MAIN ( ID, NAME ) VALUES (
11, '2.2');
INSERT INTO MAIN ( ID, NAME ) VALUES (
12, '2.2.1');
INSERT INTO MAIN ( ID, NAME ) VALUES (
13, '2.2.2');
COMMIT;
CREATE TABLE RELA
REL_ID NUMBER,
ID NUMBER,
PARENT_REL_ID NUMBER
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
1, 1, NULL);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
2, 2, 1);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
3, 4, 2);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
4, 5, 3);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
5, 7, 4);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
6, 8, 4);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
7, 9, 2);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
8, 10, 7);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
12, 7, 11);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
14, 9, 9);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
15, 10, 14);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
9, 3, 1);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
10, 4, 9);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
11, 5, 10);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
16, 11, 14);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
13, 8, 10);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
17, 12, 16);
INSERT INTO RELA ( REL_ID, ID, PARENT_REL_ID ) VALUES (
18, 13, 16);
COMMIT;
Sample Query
select rel_id,level,lpad(' ',5*level)||name,
xmlelement("rel_id", XMLAttributes(name as "name",rel_id as "rel_id")) as val
from main , rela
where main.id=rela.id
start with parent_rel_id is null
connect by prior rel_id=parent_rel_id;
Required xml output
<rel_id name="OS" rel_id="1">
     <rel_id name="Windows" rel_id="2">
          <rel_id name="1.x" rel_id="3">
               <rel_id name="1.1.x" rel_id="4">
                    <rel_id name="1.1.1.1" rel_id="5"></rel_id>
                    <rel_id name="1.2" rel_id="6"></rel_id>
               </rel_id>
          </rel_id>
          <rel_id name="2.x" rel_id="7">
               <rel_id name="2.1" rel_id="8"></rel_id>
          </rel_id>
     </rel_id>
     <rel_id name="DOS" rel_id="9">
          <rel_id name="2.x" rel_id="14">
               <rel_id name="2.1" rel_id="15"></rel_id>
               <rel_id name="2.2" rel_id="16">
                    <rel_id name="2.2.1" rel_id="17"></rel_id>
                    <rel_id name="2.2.2" rel_id="18"></rel_id>
               </rel_id>
          </rel_id>
          <rel_id name="1.x" rel_id="10">
               <rel_id name="1.1.x" rel_id="11">
                    <rel_id name="1.1.1.1" rel_id="12"></rel_id>
               </rel_id>
               <rel_id name="1.2" rel_id="13">
               </rel_id>
          </rel_id>
     </rel_id>
</rel_id>

Answer 1

will this work?
select xmlelement("rel_id",
        (select dbms_xmlgen.getxmltype
                (dbms_xmlgen.newcontextfromhierarchy
                  ('select level,
                            xmlelement("rel_id",
                             xmlattributes(a.name     as "name",
                                           b.rel_id     as "rel_id")
                       from rela     b,
                            main     a
                      where b.id     = a.id
                      start with parent_rel_id is null
                    connect by prior rel_id=parent_rel_id'))
           from dual)) xmldoc
from dual
XMLDOC
<rel_id><rel_id name="OS" rel_id="1">
  <rel_id name="Windows" rel_id="2">
    <rel_id name="1.x" rel_id="3">
      <rel_id name="1.1.x" rel_id="4">
        <rel_id name="1.1.1.1" rel_id="5"/>
        <rel_id name="1.2" rel_id="6"/>
      </rel_id>
    </rel_id>
    <rel_id name="2.x" rel_id="7">
      <rel_id name="2.1" rel_id="8"/>
    </rel_id>
  </rel_id>
  <rel_id name="DOS" rel_id="9">
    <rel_id name="1.x" rel_id="10">
      <rel_id name="1.1.x" rel_id="11">
        <rel_id name="1.1.1.1" rel_id="12"/>
      </rel_id>
      <rel_id name="1.2" rel_id="13"/>
    </rel_id>
    <rel_id name="2.x" rel_id="14">
      <rel_id name="2.1" rel_id="15"/>
      <rel_id name="2.2" rel_id="16">
        <rel_id name="2.2.1" rel_id="17"/>
        <rel_id name="2.2.2" rel_id="18"/>
      </rel_id>
    </rel_id>
  </rel_id>
</rel_id>
</rel_id>
1 row selected.