Trying to UNION two views with nested tables

Question

I am using Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Prod, and my objective is to generate some XML. What I have been doing in the past, is to create a view, and then pass that view to the DBMS_XMLQuery routine. This gives me a CLOB that I pass along to the application. A typical object would be something like:
create or replace type XML_UGroup_Member_Type as object (
     username          varchar2(128),
     group_key          varchar2(128));
create or replace type XML_UGroup_Member_List
    as table of XML_UGroup_Member_Type;
show errors
Create or replace type XML_UGroup_Type as object (
     Group_Space     varchar2(32),
     group_key     varchar2(128),
     group_name     varchar2(255),
     members          XML_UGroup_Member_List);
/This particular application will be doing stuff with group information. Pretty simply types - a group with a name and a few other keys, and a list of members. A sample view would be like:
create or replace view xml_department_ugroup_view of xml_ugroup_type
  with object identifier ( group_key ) as
  Select 'Department',
      'Department:' || orgn_code,
      'Department_' || orgn_name
      cast ( multiset (
           select  Username, person_id,
                    'Department:' ||  effective_orgn
            from directory_master dm, logins l
           where effective_orgn = dd.orgn_code
             and dm.person_id = l.owner)
            as XML_UGroup_Member_List )
         as members
   from directory_departments dd
  where dept_include = 'Y';
can select from this view, and I can pass it to the DBMS_XMLQUERY package and get a nice XML document. What I want to is essentially put several of these views together, like
Select * from XML_Department_Ugroup_View
Union
Select * from XML_Portfolio_Ugroup_View;But UNION does not work with nested tables:
ERROR at line 1: ORA-00932: inconsistent datatypes: expected - got SIMON.XML_UGROUP_MEMBER_LIST
What I was hoping to do, was to develop a set of sub views for each of the group "spaces", and then generate a view/object that includes all of the sub views, and be able to operate with that single object.
Thoughts on how to make this work, or on alternate approaches? I have considered calling XMLQuery on each of the sub views and concatenating the XML documents and returning that to the application, but I was hoping to find a more "unified" approach.

Answer 1

I modified my approach a bit - I changed the base views to return an XMLType object, like:
create or replace view xml_portfolio_ugroup_xml
as
Select XMLElement("group",
         xml_ugroup_type(
      'Portfolio',
         'Portfolio:' || coas_code || ':' || orgn_Code,
      cast ( multiset (
           select  Username,
            from directory_master dm, logins l
           where ( effective_orgn, effective_coas ) in
           where effective_orgn = dd.orgn_code
              and dm.person_id = l.owner
            as XML_UGroup_Member_List ))) as grp
   from directory_departments dd
  where dept_include = 'Y';and then I combined them with something like the following (I expect to adjust this as I learn more about the XML DB support)
create or replace view xml_ugroup_xml as
  select xmlconcat(
     (select xmlagg(grp) from xml_department_ugroup_xml),
     (select xmlagg(grp) from xml_portfolio_ugroup_xml)
     ) as GROUPS
     from dualand as I define more group branches, I can add them into the XMLConCat stanzas. I am still open to suggestions as to ways of doing this better or cleaner, but this at least gets the project moving forward again.