1. Extending the database relational model to capture more meaning;Codd;ACM Trans. Database Sys.,1979
2. Anonymous, American National Standards Institute study group on database management: Interim report;ACM SIGMOD Rec.,1975
3. Incomplete information in a relational database;Grant;Fundamenta Informaticae,1980
4. On semantic issues connected with incomplete information databases;Lipski;ACM Trans. Database Syst.,1979
5. A statistical approach to incomplete information in database systems;Wong;ACM Trans. Database Systems,1982