'local-name()' requires a singleton (or empty sequence) T-SQL Xquery - sql

Question

This is a variation/combination of these two questions, but I think I've done as they described:
Getting element's name in XPATH
XQuery [value()]: 'value()' requires a singleton (or empty sequence), found operand of type 'xdt:untypedAtomic *'
Is there a way to get the name of a node in a SQL Server 2016 T-SQL query?
I want to do this:
select
trcid,
trcXML.value(
'local-name(//*[local-name()="after"][1]/*[1])',
'varchar(32)'
) as XmlTableName,
trcXml,
trcGUID,
trcCorrID
from trace
order by trcId desc
I have explicitly included the [1] after each item in the hierarchy.
Error: XQuery [trace.trcXML.value()]: 'local-name()' requires a singleton (or empty sequence), found operand of type 'element(*,xdt:untyped) *'
Sample data:
<s1:DB2Request xmlns:s1="XYZDB2">
<s1:sync>
<s1:after identityInsert="false">
<s1:PG204AT5>
<s1:ISA06>1975111</s1:ISA06>
etc...
</s1:PG204AT5>
</s1:after>
</s1:sync>
</s1:DB2Request>
I want the table name (this is a BizTalk "update-gram") which is between the "after" and the ISA06 elements, in this case: PG204AT5.
Tested in xpathtester.com: local-name(//*[local-name()="after"][1]/* [1])
- for some reason when I save it the XPATH gets changed: http://www.xpathtester.com/xpath/2bd602d8fc7aee0484de14bf93f71ef2

Answer 1

One more set of parenthesis gives you a singleton as required:
DECLARE #t TABLE(s XML);
INSERT INTO #t(s)VALUES(N'<s1:DB2Request xmlns:s1="XYZDB2">
<s1:sync>
<s1:after identityInsert="false">
<s1:PG204AT5>
<s1:ISA06>1975111</s1:ISA06>
<somethingsomething/>
</s1:PG204AT5>
</s1:after>
</s1:sync>
</s1:DB2Request>');
select
s.value(
'local-name((//*[local-name()="after"][1]/*)[1])',
'varchar(32)'
) as XmlTableName
from #t;
Results in:
XmlTableName
PG204AT5

Answer 2

Remember that the SQL Server engine, unlike most XQuery implementations, does "pessimistic static typing" - it assumes that if things can go wrong, they will go wrong.
The expression //*[local-name()="after"][1]/* [1] is capable of returning more than one element - for example it will do so with this input
<x>
<y>
<after>
<z/>
</after>
</y>
<y>
<after>
<z/>
</after>
</y>
</x>
so the XQuery compiler infers a static type of element()*, which doesn't meet the requirement.
It's probably enough to simply do (//*[local-name()="after"]/*)[1].
Personally, I always thought pessimistic static typing was a really poor design choice, but some of the early XQuery implementors including Microsoft were very keen on it.

Answer 3

Or even simpler solution by asking explicitly first child node name of the <s1:after> element.
SQL
-- DDL and sample data population, start
DECLARE #tbl TABLE(xmldata XML);
INSERT INTO #tbl(xmldata)
VALUES(N'<s1:DB2Request xmlns:s1="XYZDB2">
<s1:sync>
<s1:after identityInsert="false">
<s1:PG204AT5>
<s1:ISA06>1975111</s1:ISA06>
<somethingsomething/>
</s1:PG204AT5>
</s1:after>
</s1:sync>
</s1:DB2Request>');
-- DDL and sample data population, end
;WITH XMLNAMESPACES (DEFAULT 'XYZDB2')
SELECT xmldata.value('local-name((/DB2Request/sync/after/child::node())[1])','VARCHAR(50)') as XmlTableName
FROM #tbl;
Output
+--------------+
| XmlTableName |
+--------------+
| PG204AT5 |
+--------------+