With the advent of the information age, the amount of digital information that is recorded and stored has been increasing at a tremendous rate. Common data formats for storage include commercial relational database engines, often interconnected via an intranet, and more recently World Wide Web sites connected via the Internet. The interconnectivity of these data sources offers the opportunity to access a vast amount of information spread over numerous data sources. Modern applications that could benefit from this wealth of digital information abound, and they range over diverse domains such as business intelligence (e.g., trade-market analysis or online web access monitoring), leisure (e.g., travel and weather), science (e.g., integration of diagnoses from nurses, doctors, and specialists about patients), libraries (e.g., multimedia online resources like museums and art collections), and education (e.g., lecture notes, syllabi, exams, and transparencies from different web sites...