Java Reference
In-Depth Information
11.1.2. FROM clause
The
FROM
clause of JPQL is by far the most important clause. It defines the domain for the
query—that is, the names for the entities that will be used in the query. If your JPQL query
is to get
Category
entities, you specify the
FROM
clause as follows:
FROM Category c
Category
is the domain that you want to query, and here you have specified
c
as an iden-
tifier of type
Category
.
Identifying the query domain: naming an entity
You specify the entity name defined for the entity using the
@Entity
annotation as the do-
main type. You can define the name for an entity using the
name
element of the
@Entity
annotation. If you don't specify the
name
element, it defaults to the name of the entity
class. The name of the entity must be unique within a persistence unit. In other words,
you can't have two entities with the same name or the persistence provider will generate a
deployment error. This makes sense because the persistence provider wouldn't be able to
identify which entity domain to use if duplicate names for entities were allowed.
In the previous example, you're assuming the
Category
entity class that we discussed in
earlier chapters doesn't define a name. If you assume that the
Category
class defines an
entity name using the
name
element as follows
@Entity(name = "CategoryEntity")
public class Category
then you must change the
FROM
clause of the query as follows:
FROM CategoryEntity c
This change is required for JPQL to map the correct entity type as defined by the annota-
tion.
