There has been tremendous interest in information integration systems that automatically gather, manipulate, and integrate data from multiple information sources on a user's behalf. Unfortunately, web sites are primarily designed for human browsing rather than for use by a computer program. Mechanically extracting their content is in general a rather difficult job if not impossible [4]. Software systems using such web information sources typically use hand-coded wrappers to extract information content of interest from web sources and translate query responses to a more structured format (e.g., relational form) before unifying them into an integrated answer to a user's query. The most recent generation of information mediator systems (e.g., Ariadne [3], CQ [5, 7], Internet Softbots [4], TSIMMIS [2]) addresses this problem by enabling a pre-wrapped set of web sources to be accessed via database-like queries.
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