Constraints on null values in relational databases
Billie S. Goldstein · Very Large Data Bases · 1981
If null values are allowed in a database, the database designer may want to restrict where they occur. Two constructs are presented for specifying where null values may occur in a relational database. Several examples demonstrate the use of these two techniques. The two constructs are shown to be almost equivalent; a criterion for exact equivalence is given. Under the same criterion, interesting special cases of both constructs are also shown to be equivalent.