The XML language is becoming the preferred means of data interchange and representation in web based applications. Usually, XML data is stored in XML repositories, which can be accessed efficiently using the standard XPath as query language. However, the specific techniques for testing these queries often ignore the functional testing. This work addresses this problem by introducing a technique based on the category-partition method for the systematic design of test input data for an XPath query. The method permits the automatic identification of categories and choices in the XPath and XML Schema implementations and the construction of constraints in order to obtain complete and valid test cases. The technique is illustrated over a practical example.