XML Query with a variable not working - sql

Question

declare #xml xml =
'<ChangeSet DBSchemaVersion="2.0.0.88" DBSyncVersion="15" ScopeId="1">
<C op="I" PK="44">
<User UserId="44" Role="3" UserName="Dummy1" />
</C>
<C op="I" PK="45">
<User UserId="45" Role="3" UserName="Dummy2" />
</C>
<C op="I" PK="46">
<User UserId="46" Role="3" UserName="Dummy3" />
</C>
</ChangeSet>'
-- this works fine
SELECT [SyncTable].Col.query('User')
FROM #xml.nodes('/ChangeSet/C[#op=''I'']') as [SyncTable](Col)
FOR XML AUTO
-- this does not work
DECLARE #tableName NVARCHAR(MAX) = 'User'
SELECT [SyncTable].Col.query('sql:variable(''#tableName'')')
FROM #xml.nodes('/ChangeSet/C[#op=''I'']') as [SyncTable](Col)
FOR XML AUTO
The first returns:
<SyncTable><User UserId="44" Role="3" UserName="Dummy1"/></SyncTable><SyncTable><User UserId="45" Role="3" UserName="Dummy2"/></SyncTable><SyncTable><User UserId="46" Role="3" UserName="Dummy3"/></SyncTable>
The second returns:
<SyncTable>User</SyncTable><SyncTable>User</SyncTable><SyncTable>User</SyncTable>
I'm trying to do this in a procedure where the table name is a parameter. Is there a way to do what I'm trying to achieve?
SQL Server 2008 R2
TIA

Answer 1

SQL variable value passed to the xquery expression as literal string, and AFAICS there is no way to inject literal string as xquery/xpath step expression.
However, there is a way around this, namely, filtering element having name equals specific string. Here, specific string is the literal string from SQL variable, for example :
DECLARE #tableName NVARCHAR(MAX) = 'User'
SELECT [SyncTable].Col.query('*[local-name()=sql:variable("#tableName")]')
FROM #xml.nodes('/ChangeSet/C[#op="I"]') as [SyncTable](Col)
FOR XML AUTO
SQL Fiddle Demo
This part *[local-name()=sql:variable("#tableName")] reads : select any child of current context element, having local-name equals value of #tableName variable.