ZEND PHP 5 Certification STUDY GUIDE

152 ” Database Programming
As you can see, we declare an inner join that creates a link between book and
book_chapter; rows are returned only if a common value for the isbn column can
be found for both tables.
Note that inner joins only work well with assertive conditions—negative conditions
often return bizarre-looking results:
SELECT * FROM book INNER JOIN book_chapter ON book.isbn book_chapter.isbn;
You would probably expect this query to return a list of all the records in the book
table that do not have a corresponding set of records in book_chapter—however,
the database engine returns a data set that contains an entry for each record in
book_chapter that does not match each record in book; the end result is, in fact, a
dataset that contains every line in book_chapter repeated many times over (the actual
size of the set depending on the number of rows between the two tables that do
have matching values for their respective isbn columns).
Outer Joins
Where inner joins restrict the results returned to those that match records in both tables,
outer joins return all records from one table, while restricting the other table to
matching records, which means that some of the columns in the results will contain
NULL values. This is a powerful, yet sometimes confusing, feature of SQL database
engines.
Left joins are a type of outer join in which every record in the left table that matches
the WHERE clause (if there is one) will be returned regardless of a match made in the
ON clause of the right table.
For example, consider the following SQL statement with a LEFT JOIN clause.
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SELECT book.title, author.last_name
FROM author
LEFT JOIN book ON book.author_id = author.id;
The table on the left is the author table because it is the table included as the primary
table for the statement in the FROM clause. The table on the right is the book table
because it is included in the JOIN clause. Since this is a LEFT JOIN and there is no