JPQL中有“SELECT 1”吗？

Question

Is there some equivalent of "SELECT 1" or "SELECT 1 FROM DUAL" for JPQL? Like a simple test query.

Answer 1

Yes, there is getResultList, which returns query results as an untyped List. And you can use setMaxResults, limiting it to 1:

query.setMaxResults(1).getResultList();

Or else you can say the following which returns a single untyped result:

query.getSingleResult()

The EntityManager API also allows creation of native queries, which may be your simplest approach:

entityManager.createNativeQuery("select 1 from sometable").getSingleResult();

For details, see http://docs.oracle.com/javaee/6/api/javax/persistence/Query.html .