How to retrieve items with hibernate clustered by to_days + COUNT(*) - mysql

Question

I am pretty new to hibernate again, so this might be a noobish question ;).
Without to_days, but clustered by timestamp it works like this:
CriteriaQuery<Tuple> query = criteriaBuilder.createQuery(Tuple.class);
Root<Session> sessionRoot = query.from(Session.class);
query.multiselect(
sessionRoot.get("time").alias("time"),
criteriaBuilder.count(sessionRoot).alias("count")
);
query.groupBy(sessionRoot.get("time"));
List<Tuple> results = this.executeQuery(query);
So I recieve:
time|count
13721938721|1
13721938722|2
13721938723|3
13721938724|4
13721938725|2
13721938726|1
13721938727|4
But this are all sessioncounts for each millisecond, but I need those clustered by day and not by timestamp: thus I use to_days in plain mysql.
In mysql I perform this query:
SELECT TO_DAYS(`time`) AS `days`, COUNT(*) as `count` FROM sessions WHERE 1 GROUP BY `days`
This gives me:
days|count
777594|123
777595|60
777596|61
777597|74
But I have no idea, yet: how to achieve the same thing with javax.persistence.criteria.CriteriaBuilder and CriteriaQuery in hibernate?

Answer 1

I dont know how to do it with criteriaBuilder, but i do know how in Hibernate 4 criteria api:
query.setProjection(
Projections.sqlProjection(
"TO_DAYS(time) as days",
new String[]{"days"},
new Type[]{StandardBasicTypes.INTEGER}
)
);
sqlProjection allows you to cast or convert data types, but careful, using a projection will only retrieve the fileds you specify in it, and the resulting list will come up like this:
List<Object[]> results = this.executeQuery(query);
But you can make hibernate do a alias match with the properties using a result transformer:
query.setResultTransformer(new AliasToBeanResultTransformer(Session.class));
and the list comes out like it normally does:
List<Session.class> results = this.executeQuery(query);
Sorry i could not provide a criteriaBuilder solution, but i hope this gets you in the right track.

Answer 2

After some investigation, it turned out, that HQL does not support TO_DAYS. Since I want to make it possible for MySQL and other databases, this is my final solution:
Query q = entityManager.createQuery("SELECT concat(day(e.time), '-', month(e.time), '-', year(e.time)) AS days, COUNT(*) FROM Event e GROUP BY concat(day(e.time), '-', month(e.time), '-', year(e.time))");
The result is:
3-5-2012|980
4-5-2012|200
10-6-2012|123
12-6-2012|144
13-11-2012|500
So afterwards I convert all ugly date strings into proper milliseconds in java and have the data, which I need.