Get max number of elements in a collection with jpql - jpa

Question

I have the following two entities (1:N) :
#Entity
public class Job {
#ManyToOne
private User user
}
and
#Entity
public class User {
#OneToMany
private Collection<Job> jobs;
}
now i want to write a named Query with jpql which gets the User(s) with the most job(s).
With the following query on the Job Entity i am able to retrieve the number of jobs for each user... but somehow i have to compare it with the number of jobs of the user which has the maximum number of jobs of all...
#NamedQuery(query="SELECT j.user, COUNT(j) FROM Job j GROUP BY j.user" )
My second idea is to write the named query on the User entity:
#NamedQuery( query="SELECT u.username FROM User u WHERE SIZE(u.jobs) = MAX ??????")
Here also i don't know how to get the number of maximum assigned jobs....
can somebody help me out?

Answer 1

To list the user order by job size, you can use following JQL:
from User order by jobs.size desc
Which generated the following SQL using Hibernate with HSQLDB:
select
user0_.id as id0_
from
User user0_
order by
(select
count(jobs1_.user_id)
from
Job jobs1_
where
user0_.id=jobs1_.user_id) desc
To find the max job size of the users, you can limit the JPA to list only top user with the most jobs with using TypedQuery.setMaxResults(1) of the query which generated the follwing SQL for HSQLDB:
select
user0_.id as id0_
from
User user0_
order by
(select
count(jobs1_.user_id)
from
Job jobs1_
where
user0_.id=jobs1_.user_id) desc limit ?

Answer 2

I found a query similar to the following successfully gave me the list of Users in descending order of the number of jobs they have:
SELECT u FROM User u LEFT JOIN FETCH u.jobs
ORDER BY u.jobs.size DESC
Then if you just get the first result, then you will be getting the User with the maximum number of jobs. Is that what you wanted?
Alternatively, to get ALL the Users that have the maximum number of jobs you could use:
SELECT u FROM User u LEFT JOIN FETCH u.jobs
WHERE u.jobs.size = (SELECT max(jobs.size) FROM User)
You probably don't need the LEFT JOIN FETCH part