I don't know how to perform a JPA criteria query that returns with a boolean output.
The goal is to have a criteria query that looks like this when executed in Oracle:
select 1 from dual where exists ( ... );
The where exists (...) part I performed with a subquery. I'm struggling with the external query.
The practical use of this is to determine whether that subquery in the exists clause returns true or false.
This is what I've written:
CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
CriteriaQuery<Object> query = criteriaBuilder.createQuery();
query.from(Boolean.class);
query.select(criteriaBuilder.literal(true));
Subquery<Location> subquery = query.subquery(Location.class);
Root<Location> subRootEntity = subquery.from(Location.class);
subquery.select(subRootEntity);
Path<?> attributePath = subRootEntity.get("State");
Predicate predicate = criteriaBuilder.equal(attributePath, criteriaBuilder.literal("TX"));
subquery.where(predicate);
query.where(criteriaBuilder.exists(subquery));
TypedQuery<Object> typedQuery = em.createQuery(query);
The last line throws an error, stating that "Boolean is not an entity". I think my issue is not knowing how to express the "from" part of the query so that the result outputs 1 or 0/ true or false - not an entity.
I know I could retrieve any entity and then check if the list of results has size of 1.
I'm asking how to get a boolean result, both to avoid the unnecessary task of retrieving those columns and also to learn how to do it.
Is this possible at all?
Thanks! Eduardo
You could do a select for one property (e.g. the ID) and set the max results returned to 1 so that you make sure the DB does not do more work than necessary (like counting all instances). Then your results list will either be empty (exists = false) or have one element (exists = true).