Matching one attribute to another using XPath/XQuery in SQL Server 2008 - sql-server

Question

Consider the XML and SQL:
declare #xml xml = '
<root>
<person id="11272">
<notes for="107">Some notes!</notes>
<item id="107" selected="1" />
</person>
<person id="77812">
<notes for="107"></notes>
<notes for="119">Hello</notes>
<item id="107" selected="0" />
<item id="119" selected="1" />
</person>
</root>'
select Row.Person.value('data(../#id)', 'int') as person_id,
Row.Person.value('data(#id)', 'int') as item_id,
Row.Person.value('data(../notes[#for=data(#id)][1])', 'varchar(max)') as notes
from #xml.nodes('/root/person/item') as Row(Person)
I end up with:
person_id item_id notes
----------- ----------- -------
77812 107 NULL
77812 119 NULL
11272 107 NULL
What I want is the 'notes' column to be pulled based on the #id attribute of the current item. If I replace [#for=data(#id)] in the selector with [#for=107] of course I get the value Some notes! in the last record. Is it possible to do this with XPath/XQuery, or am I barking up the wrong tree here? I think the problem is that
The XML is a bit awkward, yes, but I can't really change it I'm afraid.
I found one solution that works, but it feels awfully heavy for something like this.
select Item.Person.value('data(../#id)', 'int') as person_id,
Item.Person.value('data(#id)', 'int') as item_id,
Notes.Person.value('text()[1]', 'varchar(max)') as notes
from #xml.nodes('/root/person/item') as Item(Person)
inner join #xml.nodes('/root/person/notes') as Notes(Person) on
Notes.Person.value('data(#for)', 'int') = Item.Person.value('data(#id)', 'int')
and
Notes.Person.value('data(../#id)', 'int') = Item.Person.value('data(../#id)', 'int')
Update!
I figured it out! I'm new at XQuery but this works, so I'm calling it job done :) I changed the query for the notes to:
Item.Person.value('
let $id := data(#id)
return data(../notes[#for=$id])[1]
', 'varchar(max)') as notes

Answer 1

I would suggest that you do a cross apply instead of doing ../ to find a parent node. According to query plan it is a lot faster.
select P.X.value('data(#id)', 'int') as person_id,
I.X.value('data(#id)', 'int') as item_id,
I.X.value('let $id := data(#id)
return data(../notes[#for=$id])[1]', 'varchar(max)') as notes
from #xml.nodes('/root/person') as P(X)
cross apply P.X.nodes('item') as I(X)
You can even remove the ../ in the flwor with one extra cross apply gaining a bit more.
select P.X.value('#id', 'int') as person_id,
TI.id as item_id,
P.X.value('(notes[#for = sql:column("TI.id")])[1]', 'varchar(max)') as notes
from #xml.nodes('/root/person') as P(X)
cross apply P.X.nodes('item') as I(X)
cross apply (select I.X.value('#id', 'int')) as TI(id)
Comparing the queries against each other I got 67% on your query 17% on my first and 16% on the second. Note: these figures only give you a hint on what query will actually be faster in reality. Test the against your data to know for sure.