The Problem of Incomplete Information in Relational Databases

In a relational database the information is recorded as rows
in tables. However, in many practical situations the
available information is incomplete and the values for some
columns are missing. Yet few existing database management
systems allow the user to enter null values in the database.
This monograph analyses the problems raised by allowing null
values in relational databases. The analysis covers
semantical, syntactical, and computational aspects.
Algorithms for query evaluation, dependency enforcement and
updates in the presence of null values are also given.
The analysis of the computational complexity of the
algorithms suggests that from a practical point of view the
database should be stored as Horn tables, which are
generalizations of ordinary relations, allowing null values
and Horn clause-like restrictions on these null values. Horn
tables efficiently support a large class of queries,
dependencies and updates.