Although you can interact with the data in a database one row at a time, relational
databases are really all about sets. You have seen how you can create tables via queries
or subqueries, make them persistent via `insert`

statements, and bring them together via joins; this chapter explores how you can combine
multiple tables using various set operators.

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 as the *intersection* and ...

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