Question

So I have this SQL query,

<named-query name="NQ::job_exists">
<query>
select 0 from dual where exists (select * from job_queue);
</query>
</named-query>

Which I plan to use like this:

Query q = em.createNamedQuery("NQ::job_exists");
        List<Integer> results = q.getResultList();
        boolean exists = !results.isEmpty();
        return exists;

I am not very strong in SQL/JPA however, and was wondering whether there is a better way of doing it (or ways to improve it). Should I for example, write (select jq.id from job_queue jq) instead of using a star??

EDIT:This call is very performance critical in our app.

EDIT:Did some performance testing, and while the differences were almost negligible, I finally decided to go with:

select distinct null
    from dual 
       where exists (
               select null from job_queue
       );

Answer 1

IF you are using EXISTS Oracle I recommend using null:

select null 
  from dual where exists (select null from job_queue);

The following will be the one with lower cost on Oracle:

select null
  from job_queue
 where rownum = 1;

Update: To include the case when there are no rows on table you can run the following query:

select count(*)
  from (select null
          from job_queue
          where rownum = 1);

With this query you have a optimum plan and only two possible results: 1 if there are rows and 0 if there are no rows.