JDO queries often involve using SQL joins behind the scenes. You can configure Kodo 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. Kodo only supports outer joins when using SQL 92 syntax or a database-specific syntax with outer join support.
The kodo.jdbc.DBDictionary plugin accepts the the JoinSyntax property to set the system's default syntax. Syntax can be changed on a per persistence manager, query, or extent basis using the fetch configuration API, which is described in the JDO Runtime Extensions chapter.
Example 4.6. Specifying the Join Syntax at Runtime
import kodo.query.*; // for KodoQuery
import kodo.jdbc.runtime.*; // for JDBCFetchConfiguration
...
KodoQuery kq = (KodoQuery) pm.newQuery (MyClass.class, "foo == bar");
JDBCFetchConfiguration fetch = (JDBCFetchConfiguration)
kq.getFetchConfiguration ();
fetch.setJoinSyntax (fetch.JOIN_SYNTAX_SQL92);
Collection results = (Collection) kq.execute ();