Типизированные неизвестные значения: шаг к решению проблемы представления отсутствующей информации в реляционных базах данных

Abstract

The state of affairs in the area of missing information management in relational databases leaves much to be desired. The SQL standard uses the universal null value to represent missing data, and the control is based on three-valued logic, in which the null value is identified with a third boolean value. This solution is conceptually inconsistent and often results in DBMS behavior that is not intuitive. An alternative approach using typed special values leaves all handling of missing data to users. In this article, we analyze the long history of research and development that led to this situation. We come to the conclusion that no other solution could have appeared in the SQL standard due to the choice of the mechanism of the universal null value more than 50 years ago, and the alternative mechanism cannot provide system support for special values due to the use of two-valued logic. We propose a combined approach using typed special values based on three-valued logic. This approach allows you to use the semantics of data types when processing queries with conditions that include unknown data. In addition, our approach allows us to define a full-fledged three-valued logic in which a special value of a Boolean type is the third boolean value.