SQL 2008: Extract XML from nvarchar(max) - sql-server

Question

I've read all of the other posts on this subject but none are working for me. This one is the closest CONVERTING SQL NVARCHAR(MAX) TO XML
So, I've modified that code to try and fit my situation. I have a nvarchar(max) column that contains XML data as one big long string. It's not my database, so I can't change the datatype to XML. The next best thing is to create a query and cast it as XML. Here is my code:
IF OBJECT_ID('tempdb..#XML_Dummy') IS NOT NULL DROP TABLE #XML_Dummy
GO
CREATE TABLE #XML_Dummy
(
[ID] [INT] IDENTITY(1, 1)
NOT NULL ,
[XMLValue] [NVARCHAR](MAX) NULL
);
GO
INSERT INTO #XML_Dummy
(
[XMLValue]
)
select top 10 xmlcol from MyTable
GO
SELECT
b.x.value('(/Proponix/Header/SubHeader/InstrumentID)[1]', 'varchar(max)')
FROM
#XML_Dummy a
CROSS APPLY (
SELECT
CAST(CAST ([XMLValue] AS VARCHAR(MAX)) AS XML) x
) b;
IF OBJECT_ID('tempdb..#XML_Dummy') IS NOT NULL DROP TABLE #XML_Dummy;
I'm not sure why I'm getting all NULLS back. I want to return the InstrumentID. Here is an excerpt of my XML:
<?xml version="1.0"?>
<Proponix>
<Header>
Header stuff
</Header>
<SubHeader>
<InstrumentID>BS6000000001</InstrumentID>
</SubHeader>
A lot more fields
What am I missing? This is my first time working with XML in SQL server.

Answer 1

SubHeader is not a child of Header in your example. Try:
b.x.value('(/Proponix/SubHeader/InstrumentID)[1]', 'varchar(max)')

Answer 2

You try to find the SubHeader as a child of Header but it is a sibling...
Try this:
EDIT: changed #x to simulate a table with a nvarchar-column containing different xml_msg values:
EDIT2: adapted according to calls the OP showed in chat
WITH ConvertedToXML AS
(
SELECT xml_msg AsVarchar
,CAST(xml_msg AS XML) AS AsXml
FROM myTable
)
SELECT ConvertedToXML.AsVarchar
,ConvertedToXML.AsXml
,ConvertedToXML.AsXml.value('(/Proponix/SubHeader/InstrumentID)[1]', 'varchar(max)') AS InstrumentID
FROM ConvertedToXML
--old Text
declare #x table(xml_msg nvarchar(max));
insert into #x VALUES
('<?xml version="1.0"?>
<Proponix>
<Header>
Header stuff
</Header>
<SubHeader>
<InstrumentID>BS6000000001</InstrumentID>
</SubHeader>
</Proponix>')
,('<?xml version="1.0"?>
<Proponix>
<Header>
Header stuff
</Header>
<SubHeader>
<InstrumentID>BS6000000002</InstrumentID>
</SubHeader>
</Proponix>')
,('<?xml version="1.0"?>
<Proponix>
<Header>
Header stuff
</Header>
<SubHeader>
<InstrumentID>BS6000000003</InstrumentID>
</SubHeader>
</Proponix>');
WITH ConvertedToXML AS
(
SELECT TOP 10 CAST(xml_msg AS XML) AS AsXml FROM #x
)
SELECT
ConvertedToXML.AsXml.value('(/Proponix/SubHeader/InstrumentID)[1]', 'varchar(max)')
FROM ConvertedToXML