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Much of the world's data lies outside relational databases-scattered across Web sites, file systems, nonrelational databases, and legacy applications. These data sources differ in the way they organize data, in the vocabulary they use, and in their data-access or query mechanisms. These differences make it difficult to combine data from multiple sources. Virtual database (VDB) technology makes external data sources act as an extension of an enterprise's relational database system. Using VDB technology, applications can make powerful queries of data scattered over a variety of sources. The VDB gathers, structures, and integrates the data from these sources, and gives the application programmer the appearance of a single, unified RDBMS