I've already written about merging two feeds using MagpieRSS and FeedCreator php classes. However, I wasn't aware of XQuery query language that is designed to query collections of XML data. It seems similar to SQL. You can use XQuery to speed your merging and filtering of RSS / atom information (feed) easily. In this tutorial:
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You will learn the basics of XQuery processing of RSS and Atom feeds to turn a single feed into an HTML document. Then you produced a more complete solution for outputting the information in a format that suits your needs, including sorting, merging multiple feeds and even handling different feed and source information types. XQuery makes it much easier to merge and filter information from XML documents when you embed the filtering instructions right into the document that you use to generate the output format. You can use that functionality to aggregate information from RSS and Atom feeds into the format you need. In this article, look at the structure of the RSS and Atom formats and how XQuery can simplify the display of that information.