declare @i int
exec sp_xml_preparedocument @i output,
'<mydata>
<test xmlID="3" xmlData="blah blah blah"/>
<test xmlID="1" xmlData="blah"/>
</mydata>'
insert into test
select xmlID, xmlData
from OpenXml(@i, 'mydata/test')
with (xmlID int, xmlData nvarchar(30))
where xmlID not in (select xmlID from test)
update test
set test.xmlData = ox.xmlData
from OpenXml(@i, 'mydata/test')
with (xmlID int, xmlData nvarchar(30)) ox
where test.xmlID = ox.xmlID
exec sp_xml_removedocument @i