Chapter 9. Complex Joins
Outer joins and self-joins are extensions of the equijoin you learned about in Chapter 6, “Equijoins.” The outer join includes the result rows returned by the equijoin, plus extra rows where no matches are found. The self-join, as implied by the name, joins a table to itself. This type of join is useful for tables with a self-referencing relationship or when you want to determine data inconsistencies.
You will see the usefulness of these types of joins for analyzing and exploring the relationships within your data.