An extended relational algebra with control over duplicate elimination

摘要: In the pure relational model, duplicate tuples are automatically eliminated. Some real world languages such as DAPLEX, however, give users control over elimination. This paper extends model to include multiset relations, i.e., relations with tuples. It considers three formalisms for expressing queries in this model: extended algebra, tableaux, and DAPLEX. shows that, original equivalence problem conjunctive expressions algebra can be solved using is NP-complete. Finally, it demonstrates that DAPLEX have essentially same expressiveness relative expressions.