Question

I’m trying to do something which is easy as pie in PHP & Co:
SELECT COUNT(x) as numItems, AVG(y) as average, … FROM Z

In PHP I would get a simple array like [{ numItems: 0, average: 0 }] which I could use like this:

echo "Number of Items: " . $result[0]['numItems'];

Usually in JPQL you only query single objects or single columns and get Lists types, for example List<SomeEntity> or List<Long>. But what do you get, when querying multiple columns?

Answer 1

You get an Object[] (or a List<Object[]>). From the section 4.8.1 Result Type of the SELECT Clause of the JPA 1.0 specification:

The result type of the SELECT clause
is defined by the the result types of
the select_expressions contained in
it. When multiple select_expressions
are used in the SELECT clause, the
result of the query is of type
Object[], and the elements in this
result correspond in order to the
order of their specification in the
SELECT clause and in type to the
result types of each of the
select_expressions.

If you want strong typing, you can use a constructor expression in the SELECT clause. From the section 4.8.2 Constructor Expressions in the SELECT Clause:

A constructor may be used in the
SELECT list to return one or more Java
instances. The specified class is not
required to be an entity or to be
mapped to the database. The
constructor name must be fully
qualified.

If an entity class name is specified
in the SELECT NEW clause, the
resulting entity instances are in the
new state.

SELECT NEW com.acme.example.CustomerDetails(c.id, c.status, o.count)
FROM Customer c JOIN c.orders o
WHERE o.count > 100