What you specify in the result defines what form of result you get back.

• Object - this is returned if you have only a single row in the results and a single column. This is achived when you specified either UNIQUE, or just an aggregate (e.g "max(field2)")
• Object[] - this is returned if you have only a single row in the results, but more than 1 column (e.g "max(field1), avg(field2)")
• List<Object> - this is returned if you have only a single column in the result, and you don't have only aggregates in the result (e.g "field2")
• List<Object[]> - this is returned if you have more than 1 column in the result, and you don't have only aggregates in the result (e.g "field2, avg(field3)")

There are situations when you want to return a single number for a column, representing an aggregate of the values of all records. There are 5 standard JDO2 aggregate functions available. These are

• avg(val) - returns the average of "val". "val" can be a field, numeric field expression or "distinct field".
• sum(val) - returns the sum of "val". "val" can be a field, numeric field expression, or "distinct field".
• count(val) - returns the count of records of "val". "val" can be a field, or can be "this", or "distinct field".
• min(val) - returns the minimum of "val". "val" can be a field
• max(val) - returns the maximum of "val". "val" can be a field

So to utilise these you could specify something like

Query q = pm.newQuery("SELECT max(price), min(price) FROM org.jpox.samples.store.Product WHERE status == 1");

This will return a single row of results with 2 values, the maximum price and the minimum price of all products that have status code of 1.

JPOX's JDOQLQuery can provide meta data for the results of a JDOQL query before its execution. It can return a JDOQLResutSetMetaData that holds information about the number and Java-types of the returned columns of the associated query. Note that the query doesn't need to be executed but still compiled before retrieving its meta data. Here is an example usage:

Query q = pm.newQuery("SELECT this.name, sum(this.price) FROM org.jpox.samples.store.Product GROUP BY this.name");
q.compile();
JDOQLResultSetMetaData metaData = ((JDOQLQuery)q).getResultSetMetaData();
for (int i = 0; i < metaData.getExpressionCount(); i++)
{
  System.out.println("Column "+i+" is of type "+metaData.getExpressionType(i));
}

JDO 2 introduces the ability to use aggregates in queries. Here's another example using the same Product class as above, but this time looking for the maximum price of products that are CD Players. Note that the result for this particular query will be of type Double since there is a single double precision value being returned via the "result".

Declarative JDOQL :
Query query = pm.newQuery(org.jpox.samples.store.Product.class);
query.setFilter("name == \"CD Player\"");
query.setResult("max(this.price)");
List results = (List)query.execute();
Iterator iter = c.iterator();
Double max_price = (Double)iter.next();

Single-String JDOQL :
Query query = pm.newQuery("SELECT max(price) FROM org.jpox.samples.store.Product WHERE name == \"CD Player\"");
List results = (List)query.execute();
Iterator iter = c.iterator();
Double max_price = (Double)iter.next();