In a domain specific information integration system CareHaven, a top-to-down integration strategy is adopted and XML is used as integration media. The main functional components of CareHaven and the disposal of user queries are fully explained in this paper. In the fulfilling of converting XML based query request to SQLs understood by data sources, we decompose the query request to sub requests that are easier to convert. Each sub request is converted with the aid of the mapping data. And the results of all sub requests is combined into one XML documents conformed to a certain DTD.