Adding node to SQL Server XML is silently failing - sql-server

Question

Trying to figure out why this snippet does not work.
DECLARE #changeStatus XML, #changeSet XML
SELECT #changeSet = TOP 1 ChangeSet
FROM MyTable
SET #changeStatus = '<change id="' + CAST(NEWID() AS NVARCHAR(36)) + '" by="' + #authorizingUserName + '" byAccountId="' + CAST(#authorizingUserId AS NVARCHAR(36)) + '" when="' + CONVERT(NVARCHAR(50),GETUTCDATE(),127) + 'Z">'
+ '<property id="Status" name="Status" old="' + #status + '" new="Closed" />'
+ '<collections />'
+ '</change>'
-- THIS DOES NOT ERROR OUT AND DOES NOT DO ANYTHING!!
SET #changeSet.modify('insert sql:variable("#changeStatus") as last into (/changes)[1]')
The overall structure of the XML is:
<changes>
<change id="" by="" byAccountId="" when="">
<property />
<collections />
</change>
</changes>
When I run the script and check the #changeSet before and after processing, they are identical. i.e.: The #changeStatus was never added to the XML contained in #changeSet.
If the original was:
<changes>
<change id="change01" by="" byAccountId="" when="">
<property ... />
<collections />
</change>
<change id="change02" by="" byAccountId="" when="">
<property ... />
<collections />
</change>
</changes>
I expected to see:
<changes>
<change id="change01" by="" byAccountId="" when="">
<property ... />
<collections />
</change>
<change id="change02" by="" byAccountId="" when="">
<property ... />
<collections />
</change>
<change id="82ECB3C5-D3BA-4CD2-B62C-89C083E4BAA1" by="me#mydomain.com" byAccountId="1E910737-D78C-E711-9C04-00090FFE0001" when="2018-01-17T00:12:33.700Z">
<property id="Status" name="Status" old="In Review" new="Closed" />
<collections />
</change>
</changes>
Does anyone see what might be wrong?

Answer 1

You should never create an XML on string level!
This has various side-effects.
If one of the variables is NULL the whole string will be NULL
Try SELECT '<x>' + NULL + '</x>';
XML is not just text with some fancy extras. Using T-SQL's FOR XML will - for sure! - create valid XML. Now imagine one of your string contains forbidden characters...
Try SELECT '<x>' + 'The arrow "->" might appear in normal text' + '</x>';
And try to cast it to XML
Many values are not stored in readable format but in some binary format. Depending on your system's settings the conversion of such types (numbers, date and time, BLOBs) will vary. Using FOR XML will use - for sure! - the correct format in and out. You can write anything within your XML, but on another system it might fail to read it.
Try it this way:
DECLARE #changeSet XML = N'<changes/>';
DECLARE #changeStatus XML;
SET #changeStatus=
(
SELECT NEWID() AS [#id]
,'SomeAuthor' AS [#by]
,'SomeAuthorId' AS [#byAccountId]
,GETUTCDATE() AS [#when]
,'Status' AS [property/#id]
,'Status' AS [property/#name]
,'In Review' AS [property/#old]
,'Closed' AS [property/#new]
,'' AS [collections]
FOR XML PATH('change')
);
--your code works fine
SET #changeSet.modify(N'insert sql:variable("#changeStatus") as last into (/changes)[1]');
--you see the first change integrated
SELECT #changeSet;
--Now let us create one more element
SET #changeStatus=
(
SELECT NEWID() AS [#id]
,'Another' AS [#by]
,'OneMore' AS [#byAccountId]
,GETUTCDATE() AS [#when]
,'Status' AS [property/#id]
,'Status' AS [property/#name]
,'In Review' AS [property/#old]
,'Closed' AS [property/#new]
,'' AS [collections]
FOR XML PATH('change')
);
--and insert it
SET #changeSet.modify(N'insert sql:variable("#changeStatus") as last into (/changes)[1]');
--yeah, worked!
SELECT #changeSet;

Answer 2

The query you posted works in general. I ran following test:
http://rextester.com/RNX55555
However, if any of the injected parameters is null, the change will not be inserted and no error will be thrown as you try to insert NULL aka nothing.