I have chosen the paper which impressed me and influenced both myself and a large number of researchers who either were in the past (like myself) or currently are working on logical foundations of databases. This paper and several other papers by Ray Reiter have started a new way of thinking about databases - with emphasis on precise logical formulation of the hidden assumptions which are made about database content when answering database queries. It made a simple but fundamental observation that there are two equally reasonable ways of interpreting the database content: closed world assumption (facts not derivable from the database are false) and open world assumption (we cannot really say anything about facts which cannot be logically derived from the databases). Reiter observed that SQL queries interpret the database according to the closed world assumption and provided the ``missing'' axioms. His way of thinking influenced my research in my own PhD thesis and the work which I did later on deductive databases. Although Reiter's paper have not led to any ``products'' as it is common today---it was a example of a fundamental paper which influenced how people think and it is still, after 20 years a very important reference for anybody who studies databases and their logical foundations.

Reiter's work has been instrumental in subsequent research on nonmonotonic logics and different forms of negation by failure and contributed very significantly not only to databases but also to logic programming and AI.

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