An inconsistent database is a database instance that does not satisfy those integrity constraints that have been declared together with the schema of the database.
Already in the classical and seminal paper by E. F. Codd  on the relational data model it is possible to find the notions of integrity constraint and consistency of a database. The idea of consistent query answering, consisting in characterizing and computing semantically correct answers to queries in inconsistent databases, was explicitly introduced in .
A database can be seen as a model, i.e., as a simplified, abstract description, of an external reality. In the case of relational databases, one starts by choosing certain predicates of a prescribed arity. The schema of the database consists of this set of predicates, possibly attributes, which can be seen as names for the arguments of the predicates, together with an indication of the domains where the attributes can take...
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