February 2014 Archive

Extended Operators of Relational Algebra

An Algebra of Relational Operations presented the classical relational algebra, and Relational Operations on Bags introduced the modifications required to treat relations as bags of tuples rather than sets. The ideas of these two sections serve as a base for most of


Sometimes we do not want simply the average or some other aggregation of an entire column. Rather, we need to examine the tuples of a relation in groups, equivalent to the value of one or more other columns, and we aggregate only within each group. As an example, assume

Extending the Projection Operator

Well now reexamine the projection operator πL(R) introduced in Set Operations on Relations under projection. In the classical relational algebra, L is a list of (some of the) attributes of R. We extend the projection operator to allow it to compute with components of tuples as well

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