Associating Data from Separate Tables
Joins are necessary in a relational database management system because the relational model's data independence permits you to bring data from separate tables into new and unanticipated relationships. Relationships among data values become explicit when the data is manipulated—when you query the database, not when you create it.
You need not know in advance that data will join. You can discover new relationships among data in different tables by joining them. For example, you could use a join to find out whether any editor is also an author. Figure 7.1 shows two versions of the join; they differ only in syntax—more on this later.
Figure 7.1. Comparing Join Queries
This query is a little tricky. Because an ...
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