Static Type doesn't allow axis movement?

Question

Hi all, hoping someone can help me here. I have a problem, found a solution but I don't really understand it - hoping someone can explain and help me make sure I'm really solving this in the best way (or if there is a better way)
I'm trying to use value() on a parent item and I get the error
XQuery [value()]: Cannot implicitly atomize or apply 'fn:data()' to complex content elements, found type 'xs:anyType' within inferred type 'element(at{http://blah/log/Action_Touch.xsd}:Id,xs:anyType) ?'.
My repro code is below 
CREATE XML SCHEMA COLLECTION dbo.[Action_Touch]
AS N'<?xml version="1.0" encoding="utf-16"?>
<xs:schema id="Action_Touch"
targetNamespace="http://blah/log/Action_Touch.xsd"
elementFormDefault="qualified"
xmlns="http://blah/log/Action_Touch.xsd"
xmlns:mstns="http://blah/log/Action_Touch.xsd"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
>
<xs:element name="Action_Touch">
<xs:complexType>
<xs:sequence>
<xs:element name="Action" type="mstns:Action"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:complexType name="Action">
<xs:sequence>
<xs:element name="Id" type="xs:int" minOccurs="1" maxOccurs="1"/>
<xs:element name="ActionItem" type="mstns:ActionItem" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="ActionItem">
<xs:sequence>
<xs:element name="Id" type="xs:int" minOccurs="0" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
</xs:schema>
GO
create procedure dbo.test
as
declare @_Action_Touch_Xml xml (document dbo.Action_Touch)
select @_Action_Touch_Xml = N'<?xml version="1.0" encoding="utf-16"?>
<at:Action_Touch xmlns:at="http://blah/log/Action_Touch.xsd">
<at:Action>
<at:Id>15</at:Id>
<at:ActionItem>
<at:Ordinal>1</at:Ordinal>
<at:Id>28</at:Id>
</at:ActionItem>
</at:Action>
</at:Action_Touch>'
;with xmlnamespaces('http://blah/log/Action_Touch.xsd' as at
select
ai.c.value('at:Id[1]' , 'int') as Id
, ai.c.value('../at:Id[1]' , 'int') as Action_Id
from @_Action_Touch_Xml.nodes('/at:Action_Touch/at:Action/at:ActionItem') ai(c)
go
If I replace the parent referencing line with
, ai.c.value('string(../at:Id[1])' , 'int') as Action_Id
Then it is ok, but is there no better way? This seems like a hacky work around to me..
Appreciate any pointers on this. Thanks

Answer 1

Generally I avoid parent-axis (..) due to severe performance problems particularly with larger pieces of xml.  This is described in detail
here.
As is happens, parent-axis also has an issue around inferring xs:anyType, which is described in this MSDN article,
XQuery and Static Typing:
"Some operations lose type information during inference. For example, if the type of a node cannot be determined, it becomes anyType. This is not implicitly cast to any other type. These conversions occur most notably during navigation by using the parent
axis. You should avoid using such operations and rewrite the query, if the expression will create a static type error."
The solution is to rewrite the query, using CROSS|OUTER apply to drill into the xml left to right, eg
;WITH XMLNAMESPACES('http://blah/log/Action_Touch.xsd' AS at )
SELECT
ai.c.value('at:Id[1]', 'INT') AS ActionItem_Id
, a.c.value('at:Id[1]', 'INT') AS Action_Id
FROM @_Action_Touch_Xml.nodes('/at:Action_Touch/at:Action') a(c)
CROSS APPLY a.c.nodes('at:ActionItem') ai(c)