1. A relational model for large shared data banks;Codd;Communs. ACM,1970
2. A consideration on normal form of not-necessarily-normalized relations in the relational data model;Makinouchi,1977
3. Remarks on the algebra of non first normal form relations;Jaeschke,1982
4. Nested relational structures;Thomas,1986
5. Non first normal form relations to represent hierarchically organized data;Abiteboul,1984