Query Relaxation for XML Databases

由于与HTML相比，可扩展标记语言(XML)具有更强的表现力，因此它已经成为Web应用程序数据表示的标准格式。然而，在基于内容的检索中，如果数据库内容与查询条件不完全匹配，或者如果查询条件没有很好地呈现，系统将返回空答案。用户必须重新制定并重新提交查询以进行处理。因此，当查询条件不是确切已知的(例如，将图像与一组特征或文档与一组关键字匹配)时，用户通常以一般的方式指定它们以避免空答案。这种方法的缺点是系统可能返回太多的答案，有时甚至是不相关的答案。为了解决这一问题，将开发一种系统的、可扩展的基于知识(源自XML数据)的松弛方法来放宽查询条件(包括值和/或结构)。此外，用户可以提供诸如要松弛的属性的顺序、松弛级别、不可松弛的属性列表、答案数量等松弛控制。结果，这种技术提供了集中和受控的松弛，从而产生更准确和相关的答案。在该方案中，将扩展现有的XML查询语言以支持查询松弛和松弛控制构造，并将开发技术来将来自XML数据的相似数据树聚集成层次结构以用作索引来引导松弛。

Due to the expressiveness of Extensible Markup Language (XML) as compared to HTML, XML has become a standard format in data presentation for the web applications. However, in content based retrieval, if the database content does not match with the query conditions exactly, or if the query conditions are not well presented, the system returns null answer. The user must reformulate and resubmit the query for processing. Therefore, when the query conditions are not known exactly (e.g. matching an image with a set of features or a document with a set of key words), the user often specifies them in a general way to avoid a null answer. The drawback of such an approach is that the system may return too many and sometimes irrelevant answers. To remedy this problem, a systematic and scalable knowledge based (derived from the XML data) relaxation methodology will be developed to relax the query conditions (both in value and / or structure). Further, the user can provide such relaxation control as the order of attributes to be relaxed, relaxation level, the unrelaxable attribute list, number of answers, etc. As a result, such technique provides a focused and controlled relaxation, and thus yields more accurate and relevant answers.In this proposal, the existing XML query language will be extended to support query relaxation and relaxation control constructs, and techniques will be developed to cluster similar data trees from XML data into a hierarchy to serve as an index to guide relaxation.