Equivalence of SQL queries in presence of embedded dependencies
Rada Y. Chirkova, Michael Genesereth · 2009
We consider the problem of finding equivalent minimal-size reformulations of SQL queries in presence of embedded dependencies [1]. Our focus is on select-project-join (SPJ) queries with equality comparisons, also known as safe conjunctive (CQ) queries, possibly with grouping and aggregation. For SPJ queries, the semantics of the SQL standard treats query answers as multisets (bags), whereas the stored relations are treated either as sets, which is called bag-set semantics, or as bags, which is called bag semantics. (Under set semantics, both query answers and stored relations are treated as sets.)