Semistructured data may be irregular and incomplete and does not necessarily conform to a fixed schema. As with structured data, it is often desirable to maintain a history of changes to data, and to query over both the data and the changes. Representing and querying changes in semistructured data is more difficult than in structured data due to the irregularity and lack of schema. We present a model for representing changes in semistructured data and a language for querying over these changes. An important feature of our approach is that we represent and query changes directly as annotations on the affected data, instead of indirectly as the difference between database states. We describe the implementation of our model and query language. We also describe the design and implementation of a query subscription service that permits users to subscribe to changes in semistructured information sources.
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