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Many database applications and OLAP tools dynamically generate SQL queries involving join operators and aggregate functions and send these queries to a database server for execution. This dynamically generated SQL code normally assumes the underlying tables and columns are clean and lacks the necessary robustness to deal with foreign keys with null and invalid or undefined values that are ubiquitous in databases with inconsistent or incomplete content. The outcome is that at query time, several issues arise mostly as inconsistencies in answer sets, difficult to detect and explain by users of OLAP tools.
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