Oracle 18c

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create table item (customer_id, item_xml) as
select 1, xmltype('<items>
  <item>Apple</item>
</items>')
from dual
union all
select 2, xmltype('<items xmlns:xml="http://www.w3.org/XML/1998/namespace">
  <item>Banana</item>
</items>')
from dual;
2 rows affected

select * from item;


CUSTOMER_ID
ITEM_XML


1
<items>
  <item>Apple</item>
</items>


2
<items xmlns:xml="http://www.w3.org/XML/1998/namespace">
  <item>Banana</item>
</items>



UPDATE item SET item_xml = XMLQuery('
  copy $n := .
  modify (  
    for $i in $n/items/item[$item][not(@xml:id)]
      return insert node attribute xml:id {$id} into $i
    )
  return $n'
  passing item_xml, 'Apple' as "item", 123 as "id"
  returning content
)
where customer_id = 1;
1 rows affected

UPDATE item SET item_xml = XMLQuery('
  copy $n := .
  modify (  
    for $i in $n/items/item[$item][not(@xml:id)]
      return insert node attribute xml:id {$id} into $i
    )
  return $n'
  passing item_xml, 'Banana' as "item", 456 as "id"
  returning content
)
where customer_id = 2;
1 rows affected

select item_xml from item where customer_id = 1;


ITEM_XML


<items>
  <item xml:id="123" xmlns:xml="http://www.w3.org/XML/1998/namespace">Apple</item>
</items>



select item_xml from item;


ITEM_XML


<items>
  <item xml:id="123" xmlns:xml="http://www.w3.org/XML/1998/namespace">Apple</item>
</items>


<items xmlns:xml="http://www.w3.org/XML/1998/namespace">
  <item xml:id="456" xmlns:xml="http://www.w3.org/XML/1998/namespace">Banana</item>
</items>

CUSTOMER_ID	ITEM_XML
1	<items> <item>Apple</item> </items>
2	<items xmlns:xml="http://www.w3.org/XML/1998/namespace"> <item>Banana</item> </items>