JPQL - EJB 3 in Action - page 440

SELECT u
FROM User u INNER JOIN u.Category c
WHERE u.userId LIKE ?1
The INNER clause is optional. Remember that INNER JOIN is the default when you use
the JOIN operator by itself, without specifying INNER or OUTER . Now let's move to the
other end of the spectrum: outer joins.
Outer joins
Outer joins allow you to retrieve additional entities that don't match the JOIN conditions
when associations between entities are optional. Outer joins are particularly useful in re-
porting. Assume that there's an optional relationship between User and Category and
you want to generate a report that prints all the category names for the user. If the user
doesn't have any categories, then you want to print NULL . If you specify the user on the left
side of the JOIN , you can use either the LEFT JOIN or LEFT OUTER JOIN keyword
phrases with a JPQL query as follows:
SELECT u
FROM User u LEFT OUTER JOIN u.Category c
WHERE u.userId like ?1
This will also retrieve User entities that don't have a matching Category , as well as
those who do. It's worth noting that if an outer join isn't used, the query would only re-
trieve the users with the matching category but would fail to retrieve users that didn't have
a matching category.
Although INNER JOIN and OUTER JOIN are the most common, are there any other
types of JOIN s supported by JPQL? We're glad you asked! The answer is yes, and next
we'll look at fetch and theta joins.
Fetch joins
In a typical business application, you may want to query for a particular entity but also
retrieve its associated entities at the same time. For example, when you retrieve a Bid in
the ActionBazaar system, you want to eagerly load and initialize the associated instance of
bidder . You can use a FETCH JOIN clause in JPQL to retrieve an associated entity as a
side effect of the retrieval of an entity: