TABLE OF CONTENTS 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 departments, and so forth. Each division budget is the sum of its department's budgets, and the sum of the division budgets is the total for the whole company again. We would not be sure what to do if a department belonged to two different divisions, because that would be a covering and not a partition.
A common problem in data processing is classifying things by the way they fall into a range on a numeric or alphabetic scale. The best approach to translating a code into a value when ranges are involved is to set up a table with the high and the...
