On semantic issues connected with keys in relational databases permitting null values

The assumption on forbidden null values in primary keys is one of the common assumptions in relational database theory and database practice: None of the attributes of the primary key may ever obtain an undefined, unknown value, since otherwise we would not know what entity a tuple with an undefined value of the primary key represents. In this paper, a new approach based on distinguished tuples is presented. Key sets, a generalization of keys and existence constraints, will be considered.