SQL-like and Quel-like correlation queries with aggregates revis-ited

Recent transformation algorithms for speeding up evaluation of nested SQL-like queries with aggregates are reviewed with respect to the correctness for aggregates over empty sets. It turns out that for a particular subset of such queries these algorithms fail to compute consistent answers. Unfortunately there seems to be no way that these transformations can be made correctly under all circumstances. Also the algorithms for QL"EL are reexamined, likewise revealing a bug for nesting levels greater than one. However, in contrast, this error can easily be fixed. Moreover, as a valuable by-product from the user's view, for a specific subset of QUEL-queries with aggregates a simple semantics can be associated a posteriori. This report was prepared while the author was on leave from the Technische Universitaet Muenchen. lnstitut fuer lnformatik. West Germany. Die Arbeit wurde mit Unterstuetzung eines Stipendiums des Wissenschaftsausschuss der XATO ueber den DAAD ermoeglkht.