1. Equivalences of relational expressions;Aho;SIAM J. Comput.,1979
2. Efficient optimization of a class of relational expressions;Aho,1978
3. A formal approach to null values in database relations;Biskup,1979
4. Optimal implementation of conjunctive queries in relational databases;Chandra,1977
5. A relational model for large shared data banks;Codd;Comm. ACM,1970