Although you can interact with the data in a database one row at a time, relational databases are really all about sets. This chapter explores how you can combine multiple result sets using various set operators. After a quick overview of set theory, I’ll demonstrate how to use the set operators union, intersect, and except to blend multiple data sets together.
Set Theory Primer
In many parts of the world, basic set theory is included in elementary-level math curriculums. Perhaps you recall looking at something like what is shown in Figure 6-1.
Figure 6-1. The union operation
The shaded area in Figure 6-1 represents the union of sets A and B, which is the combination of the two sets (with any overlapping regions included only once). Is this starting to look familiar? If so, then you’ll finally get a chance to put that knowledge to use; if not, don’t worry, because it’s easy to visualize using a couple of diagrams.
Using circles to represent two data sets (A and B), imagine a subset of data that is common to both sets; this common data is represented by the overlapping area shown in Figure 6-1. Since set theory is rather uninteresting without an overlap between data sets, I use the same diagram to illustrate each set operation. There is another set operation that is concerned only with the overlap between two data sets; this operation is known ...
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