JPA does not seem to be db agnostic

The following jpa code works with mysql but not with objectdb. Why would this happen? The provider the both jpas is the same: EclipseLink. The link for the one used with mysql is org.eclipse.persistence.jpa.PersistenceProvider; the one for objectdb is com.objectdb.jpa.Provider (per the tutorials).

    public Friendship getFriendship(String username) {
        CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
        CriteriaQuery<Friendship> query = criteriaBuilder.createQuery(Friendship.class);
        Root<Friendship> root = query.from(Friendship.class);
        Path<String> path = root.<String>get("username");
        query.where(criteriaBuilder.equal(path, username));
        return em.createQuery(query).getSingleResult();
    }

A valid JPQL query must include SELECT, otherwise the query is incomplete.

Some JPA implementations extend the standard JPQL by supporting queries with no SELECT.