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Materialized views are a natural embodiment of the ideas of precomputation and caching in databases. Instead of computing a query from scratch, a system can use results that have already been computed, stored, and kept in sync with database updates. The ability of materialized views to speed up queries benefits most database applications, ranging from traditional querying and reporting to web database caching, online analytical processing, and data mining. By reducing dependency on the availability of base data, materialized views have also laid much of the foundation for information integration and data warehousing systems. The database tradition of declarative querying distinguishes materialized views from generic applications of precomputation and caching in other contexts, and makes materialized views especially interesting, powerful, and challenging at the same time. Study of materialized views has generated a rich research literature and mature commercial implementations, aimed t providing efficient, effective, automated, and general solutions to the selection, use, and maintenance of materialized views. This monograph provides an accessible introduction and reference to materialized views, explains its core ideas, highlights its recent developments, and points out its sometimes subtle connections to other research topics in databases.