Silent XQuery failures

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A newsgroup post I read earlier today reminded me that there are cases where failure to get the expected result when using XQuery can sometimes be difficult to diagnose.

Here’s what the poster was doing

declare @xml xml

set @xml = '<root><test></test></root>'

set @xml.modify('replace value of (/root/test/text())[1] with "test

new value"')

select @xml

This user expected to get something like <root><test>test new value</test></root> but instead, the resulting instance looked like <root><test /></root>.

The server never gave any error or warning, yet the XML instance appeared to be untouched.

The problem, as you might have guessed already, is that XPath expression (/root/test/text())[1] returns the empty sequence. In the original instance, ‘test’ is an empty element. It has no children at all. Therefore the expression (/root/test/text())[1] doesn’t point to any existing node. The solution to this problem is to insert a new test node inside the ‘test’ element, like this

set @xml.modify('insert text{"test new value"} as first into (/root/test)[1] ')

 

In general, when you use the modify() method and the target XPath expression returns the empty sequence, nothing happens. The only exception to this rule is when the server can statically determine that the result will be empty. That’s when XML schemas come in handy.

For example, let’s look at the following example

DECLARE @x XML

SET @x = '<root><x/><y/><z/></root>'

SET @x.modify('delete /root[1]/a[1]')

 

Let’s suppose I made a typo and instead of typing ‘z’ in my XPath expression I typed ‘a’. The query doesn’t do anything because with this particular XML instance the XPath expression /root[1]/a[1] returns the empty sequence (it doesn’t point to any existing node). There is no way to know that before executing the query. Statically, the type of this XPath expression is element(a, xdt:untyped)?

 

Now, let’s try something similar when a schema collection is associated with the XML variable.

 

CREATE XML SCHEMA COLLECTION SC AS '

<schema xmlns="https://www.w3.org/2001/XMLSchema">

      <element name="root">

            <complexType>

                  <sequence>

                        <element name="x" type="string" minOccurs="0"/>

                        <element name="y" type="string" minOccurs="0"/>

                        <element name="z" type="string" minOccurs="0"/>

                  </sequence>

            </complexType>

      </element>

</schema>'

go

DECLARE @x XML(SC)

SET @x = '<root><x/><y/><z/></root>'

SET @x.modify(' delete /root[1]/a[1]')

 

This time the query fails with the following error message:

XQuery [modify()]: There is no element named 'a' in the type 'element(root,#anonymous) ?'.

 

The server was able to use the schema to determine that my XPath expression will never point to an existing node and sent me a helpful error message. Even though typing your data doesn’t mean you’ll catch all those mistakes, it still helps when debugging XML queries.

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