Nowadays, when a user wants to get information, they might need to retrieve data from more than one databases. In this case, data integration becomes an important issue. One way to solve this problem is by having query rewriting. It has been known that when these databases are actually storing XML data, the simple query rewriting techniques like they are widely known for Relational Databases cannot be used. 
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Few existing works have proposed new algorithms to perform query rewriting in multiple resources. However, the problem occurs when the data is actually stored in heterogeneous repositories. For example, one repository is Native XML Data, where the data can be retrieved using XQuery and another repository is XML-Enabled Database, where the data can be retrieved using SQL/XML.
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This work aims to propose a query rewriting technique that not only consider the hierarchical nature of the XML data, but also the way they are stored.
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