Java Reference
In-Depth Information
SOME
is just an alias (or a synonym) for
ANY;
therefore, it can be used anywhere
ANY
can
be used.
11.1.5. Joining entities
If you've used relational databases and SQL, you must have some experience with the
JOIN
operator. You can use
JOIN
to create a Cartesian product between two entities.
Normally you provide a
WHERE
clause to specify the
JOIN
condition between entities in-
stead of just creating a Cartesian product.
You have to specify the entities in the
FROM
clause to create a
JOIN
between two or more
entities. The two entities are joined based on either their relationships or any arbitrary per-
sistence fields. When two entities are joined, you may decide to retrieve results that match
the
JOIN
conditions. For example, suppose you join
Category
and
Item
using the re-
lationships between them and retrieve only entities that match the
JOIN
condition. Such
joins are known as inner joins. Conversely, suppose you need to retrieve results that satisfy
the
JOIN
conditions but also include entities from one side of the domain that don't have
matching entities on the other side. For example, you may want to retrieve all instances of
Category
even if there's no matching instance of
Item
. This type of join is called an
outer join. Note that an outer join can be left, right, or both.
Let's first look at some examples of the most common types of joins: inner joins and outer
joins. After this, we'll look at the less-commonly used (though sometimes useful) fetch and
theta joins.
Inner joins
A common situation in applications is the need to join two or more entities based on
something shared in their relationship. Here's the syntax for
INNER JOIN
:
[INNER] JOIN join_association_path_expression [AS]
identification_variable
In ActionBazaar,
Category
and
User
entities have a many-to-one association. To re-
trieve all users who match a specific criterion, you could try this query:
