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Database integration builds on tables coming from multiple databases by creating a single view of all these data. Each database has different tables, columns with similar content across databases and different referential integrity constraints. Thus, a query in an integrated database is likely to involve tables and columns with referential integrity errors. In a data warehouse environment, even though the ETL processes take care of the referential integrity errors, in many scenarios this is generally done by including 'Dummy' records in the dimension tables used to relate to the fact tables with referential errors. When two tables are joined, and aggregations are computed, the tuples with an undefined foreign key value are aggregated in a group marked as undefined effectively discarding potentially valuable information.
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