GROUP-BY

The group-by operator is an unusual relational operator: it partitions the relation into-disjoint tuple sets and then aggregates over each set. It groups the facts characterized by the same dimension values together. The standard SQL-group-by operator does not allow a direct construction of aggregation over computed categories. For this reason its generalization (especially referring to the OLAP applications) was proposed in Gray et al. (1996, 1997), and subsequently in Lehner (1998), Hurtado, Mendelzon, & Vaisman (1999), and Nguyen, Tjoa, & Wagner (2000).