Modify XML values identified through cross apply - sql

Question

I've got a data issue with some values stored in an XML column in a database. I've reproduced the problem as the following example:
Setup Script:
create table XMLTest
(
[XML] xml
)
--A row with two duff entries
insert XMLTest values ('
<root>
<item>
<flag>false</flag>
<frac>0.5</frac>
</item>
<item>
<flag>false</flag>
<frac>0</frac>
</item>
<item>
<flag>false</flag>
<frac>0.5</frac>
</item>
<item>
<flag>true</flag>
<frac>0.5</frac>
</item>
</root>
')
In the XML portion the incorrect entries are those with <flag>false</flag> and <frac>0.5</frac> as the value of flag should be true for non-zero frac values.
The following SQL identifies the XML item nodes that require update:
select
i.query('.')
from
XMLTest
cross apply xml.nodes('root/item[flag="false" and frac > 0]') x(i)
I want to do an update to correct these nodes, but I don't see how to modify the item elements identified by a cross apply. I saw the update as looking something like this:
update t
set
x.i.modify('replace value of (flag/text())[1] with "true"')
from
XMLTest t
cross apply xml.nodes('root/item[flag="false" and frac > 0]') x(i)
However this isn't working: I get the error "Incorrect syntax near 'modify'".
Can this be done through this method?
I know an alternative would be to do a string replace on the xml column, but I don't like that as being a bit unsubtle (and I'm not confident it wouldn't break things in my real-word problem)

Answer 1

It is not possible to update the one XML instance in more than one place at a time so you have to do the updates in a loop until you are done.
From http://msdn.microsoft.com/en-us/library/ms190675.aspx "Expression1: Identifies a node whose value is to be updated. It must identify only a single node."
-- While there are rows that needs to be updated
while exists(select *
from XMLTest
where [XML].exist('root/item[flag="false" and frac > 0]') = 1)
begin
-- Update the first occurence in each XML instance
update XMLTest set
[XML].modify('replace value of (root/item[flag="false" and frac > 0]/flag/text())[1] with "true"')
where xml.exist('root/item[flag="false" and frac > 0]') = 1
end