Partitioning Data in Queries
This section is concerned with how to break the data in SQL into meaningful subsets that can then be presented to the user or passed along for further reduction.
We need to define some basic set operations. A covering is a collection of subsets, drawn from a set, whose union is the original set. A partition is a covering whose subsets do not intersect each other. Cutting up a pizza is a partitioning; smothering it in two layers of pepperoni slices is a covering.
Partitions are the basis for most reports. The property that makes partitions useful for reports is aggregation: the whole is the sum of its parts. For example, a company budget is broken into divisions, divisions are broken into ...