C-tables; Extended relations
A conditional table  generalizes relations in two ways. First, in the entries in the columns, variables, representing unknown values, are allowed in addition to the usual constants. The second generalization is that each tuple is associated with a condition, which is a Boolean combination of atoms of the form x = y, x = a, a = b, for x, y null values (variables), and a, b constants. A conditional table essentially represents an existentially quantified function free first order theory.
- 1.Abiteboul S, Duschka OM. Complexity of answering queries using materialized views. In: Proceedings of the 17th ACM SIGACT-SIGMOD-SIGART Symposium on Principles of Database Systems; 1998. p. 254–63.Google Scholar
- 3.Grahne G, Mendelzon AO. Tableau techniques for querying information sources through global schemas. In: Proceedings of the 7th International Conference on Database Theory; 1999. p. 332–47.Google Scholar