SQL queries without a candidate class are treated as projections of
column data. If you select a single column, the query returns
a list of Objects. If you select multiple
columns, it returns a list of Object[]s.
In either case, each column value is obtained using the
java.sql.ResultSet.getObject method. The
following example demonstrates a query for the values of the
ISBN and VERS columns of all
MAG table records, using the data model we
defined in Section 17.2, “Retrieving Persistent Objects with SQL”.
Example 17.5. Column Projection
Query query = pm.newQuery ("javax.jdo.query.SQL",
"SELECT ISBN, VERS FROM MAG");
List results = (List) query.execute ();
for (Iterator itr = results.iterator (); itr.hasNext ();)
{
Object[] data = (Object[]) results.next ();
processISBNAndVersion (data[0], data[1]);
}
query.close (results);
Notice that in the code above, we did not set a candidate class. Therefore, the query is treated as a projection.
Our discussion of JDOQL query result classes in
Section 11.8, “Result Class” also
applies to SQL queries. As with JDOQL queries, SQL queries can
automatically pack their results into objects of a specified type.
JDO uses the java.sql.ResultSetMetaData.getColumnLabel
method to match each column alias to the result class'
public fields and JavaBean setter methods. Here is a modification of
our example above that packs the selected column values into JavaBean
instances.
Example 17.6. Result Class
public class Identity
{
private String id;
private int versionNumber;
public void setId (String id)
{
this.id = id;
}
public String getId ()
{
return id;
}
public void setVersionNumber (int versionNumber)
{
this.versionNumber = versionNumber;
}
public int getVersionNumber ()
{
return versionNumber;
}
}
Query query = pm.newQuery ("javax.jdo.query.SQL",
"SELECT ISBN AS id, VERS AS versionNumber FROM MAG");
query.setResultClass (Identity.class);
List results = (List) query.execute ();
for (Iterator itr = results.iterator (); itr.hasNext ();)
processIdentity ((Identity) itr.next ());
query.close (results);
|
|