I've got a data schema which represents an xml document like:
...
<purchases>
<purchase>
     <category></category>
     <item></item>
     <price></price>
</purchase>
</purchases>
...

There can be an unlimited number of purchases.

I'd like to create a repeating table which lists all the categories
and sums up the prices.  Something similar to the SQL statement:
Select category, sum(price) from purchases group by category.

Is this possible with my existing schema?

I've though about adding another repeating element:
<category_name></category_name>

And adding categories as purchase/category is modifed, then I can
create a repeating table of category_name, with an expression box to
sum up the totals.  This is where the problem came up.  With a schema
like I've shown, I can't seem to get what I want.  I tried an XPath
expression like sum(../../purchase[category=category_name]/price), but
infopath complains that purchase/category_name cannot be found.  It
appears that items referenced in the []'s are relative to purchase?
instead of the initial context of the xpath.  Can anyone help?