How to return Collections and other Types in EJB QL ?

In this tutorial we will learn how to create new Type safe instances of an Objects simply using the EJB QL in your Query.

By default, the select clause picks which objects and properties to return in the query result set. Check this example:

select mate
from Cat as cat
    inner join cat.mate as mate

The above query will select mates of other Cats.

Queries can return properties of any value type including properties of component type:

select cat.name from DomesticCat cat
where cat.name like 'fri%'
select cust.name.firstName from Customer as cust

Queries can return multiple objects and/or properties as an array of type Object[]:

select mother, offspr, mate.name
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offsp

Or as a java.uti.List:

select new list(mother, offspr, mate.name)
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

Or – assuming that the class Family has an appropriate constructor – as an actual typesafe Java object:

select new Family(mother, mate, offspr)
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

You can assign aliases to selected expressions using as:

select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat

This is most useful when used together with select new map:

select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
from Cat cat