Object queries often involve using SQL joins behind the scenes. You can
configure OpenJPA to use either SQL 92-style join syntax, in which joins are
placed in the SQL FROM clause, the traditional join syntax, in which join
criteria are part of the WHERE clause, or a database-specific join syntax
mandated by the
DBDictionary. OpenJPA only supports outer joins when using
SQL 92 syntax or a database-specific syntax with outer join support.
The
openjpa.jdbc.DBDictionary plugin accepts the
JoinSyntax property to set the system's default syntax. The available
values are:
traditional: Traditional SQL join syntax; outer joins are
not supported.
database: The database's native join syntax. Databases that
do not have a native syntax will default to one of the other options.
sql92: ANSI SQL92 join syntax. Outer joins are supported.
Not all databases support this syntax.
You can change the join syntax at runtime through the OpenJPA fetch configuration API, which is described in Chapter 9, Runtime Extensions .
Example 4.8. Specifying the Join Syntax Default
<property name="openjpa.jdbc.DBDictionary" value="JoinSyntax=sql92"/>
Example 4.9. Specifying the Join Syntax at Runtime
import org.apache.openjpa.persistence.jdbc.*;
...
Query q = em.createQuery("select m from Magazine m where m.title = 'JDJ'");
OpenJPAQuery kq = OpenJPAPersistence.cast(q);
JDBCFetchPlan fetch = (JDBCFetchPlan) kq.getFetchPlan();
fetch.setJoinSyntax(JoinSyntax.SQL92);
List results = q.getResultList();