使用SUM（）的Java EE命名查询不起作用

Question

I've created a simple @NamedQuery:

@NamedQuery(name = "Etap.findSum", query = "SELECT e.czas FROM Etap e WHERE e.stan=1")

which is working fine. The field e.czas was mapped as an Integer and is of type INTEGER in DB2 database. Now when I am trying to change it in that way:

@NamedQuery(name = "Etap.findSum", query = "SELECT SUM(e.czas) FROM Etap e WHERE e.stan=1")

it fails, and what is more, the editor does not give a hint to choose the filed e.czas as a parameter of the query. When I change the type of the filed czas to int , the hint is given, but the query still doesn't work (it works when called from IBM Data Studio console).

The method where I call the query is listed below (I always get -1 as a result):

public Integer suma(){
        try{
            Query q = em.createNamedQuery("Etap.findSum");
            Integer suma = (Integer)q.getSingleResult();
            return suma;
        }
        catch(Exception e)
        {
            return -1;
        }

What am I doing wrong in this case?

Answer 1

This is probably because the ORM is auto-widening the result in anticipation of the result turning into a Long , and not an Integer .
Note that if the source column is a Short (equivalent), then SUM(...) widens the result to an Integer in all cases . The documentation doesn't explicitly mention what happens in the case of an Integer source column, but I'd imagine the db auto-widens it in the case the result is sufficiently large.
The ORM then has to account for this potential behavior by making sure it can handle a long - so it casts it to that type, unless the result is specifically stated to be safe (by casting it to Integer on the db).