Chapter 7. Joins
All the queries so far have retrieved rows from a single table. This chapter explains how to use joins to retrieve rows from multiple tables simultaneously. Recall from “Relationships” in Chapter 2 that a relationship is an association established between common columns in two tables. A join is a table operation that uses related columns to combine rows from two input tables into one result table. You can chain joins to retrieve rows from an unlimited number of tables.
Why do joins matter? The most important database information isn’t so much stored in the rows of individual tables; rather, it’s the implied relationships between sets of related rows. In the sample database, for example, the individual rows of the tables