Best way to write a named SQL query that returns if row exists?

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

Why not just do:

select count(*) as JobCount from job_queue

If JobCount = 0, then there's your answer!

Answer 2

If you do an "exists" then it will stop looking as soon as it finds a match. This can stop it from doing a full table scan. Same with TOP 1 if you don't have an ORDER BY. If you do a TOP 1 ID and ID is in an index it might use the index and not even go to the table at all. Stopping the full table scan is where the biggest saving in performance is.

Another small savings is that if you do "SELECT 1" or "SELECT COUNT(1)" instead of "SELECT * " or "SELECT COUNT(*)" it saves the work of getting the table structure.

So I would go with:

SELECT TOP 1 1 AS Found
FROM job_queue

Then:
return !results.isEmpty();

This is the least amount of work that I can think of.

For Oracle it would be:

SELECT 1 
FROM job_queue
WHERE rownum<2;

Or:

Set Rowcount 1
SELECT 1 
FROM job_queue

Answer 3

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.