Issue with nested SQL query

I’m trying to implement a nested SQL query:

                            <sql:query>

                                select distinct(total_operators.organization_name) level1 from total_countries, total_operators, operators where total_operators.id_country=total_countries.id_country and total_operators.id_operator=operators.id_operator and total_countries.country_area = <sql:param type="xs:string" select="/country"/>

                                order by level1

                            </sql:query>

                            <sql:result-set>

                                <sql:row-iterator>

                               <organization>

                                               <organizationname>

                                                               <sql:get-column-value type="xs:string" column="level1"/>

                                               </organizationname>

                                               <networks>

                                                                <sql:execute>

                                                                              <sql:query>

                                                                                                              select distinct(total_operators.network_name) level2 from total_countries, total_operators, operators where total_operators.id_country=total_countries.id_country and total_operators.id_operator=operators.id_operator and total_countries.country_area = <sql:param type="xs:string" select="/country"/> and total_operators.organization_name = <sql:param type="xs:string"><sql:get-column-value type="xs:string" column="level1"/></sql:param>  order by level2

I followed the example in the documentation but I have got the following error:

2008-12-10 14:52:35,171 ERROR [org.orbeon.oxf.webapp.ProcessorService] Exception at line 45 of oxf:/apps/newsrv/summary/detail-operators-get.xpl

org.orbeon.oxf.common.ValidationException: line 45 of oxf:/apps/newsrv/summary/detail-operators-get.xpl: Error tag name "sql:get-column-value" is not allowed. Possible tag names are: <get-column>, and more(schema: http://orbeon.org/oxf/xml/sql)

oxf:/apps/newsrv/summary/detail-operators-get.xpl, line 45, column -1: Error tag name "sql:get-column-value" is not allowed. Possible tag names are: <get-column>, and more(schema: http://orbeon.org/oxf/xml/sql)

I cannot use the sql-group syntax.

Regards.

Alessandro