Typed Unknown Values: A Step towards Solving the Problem of Missing Data Representation in Relational Databases

The state of affairs in the field of missing data management in relational databases leaves much to be desired. The SQL standard uses the universal null value to represent missing data, and the management is based on three-valued logic, in which the null value is identified with the third Boolean value. This solution is conceptually inconsistent and often results in DBMS behavior that is not intuitive. An alternative approach based on typed special values leaves all handling of missing data to users. In this paper, 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 because of the choice of the mechanism of universal null value more than 50 years ago, whereas 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 one to use the semantics of data types when processing queries with conditions that involve unknown data. In addition, our approach makes it possible to define a full-fledged three-valued logic in which a special value of the Boolean type is the third Boolean value.