How do I count the number of rows returned by subquery?

Question

I want to do something like this:

select count(*) from (select ...)

(As it would be in SQL), but in JPA.

Any ideas on how I would do it?

Answer 1

This should do the trick (If you want to use JPA criteria API):

CriteriaBuilder cb = getEntityManager().getCriteriaBuilder();  
CriteriaQuery<Long> query = cb.createQuery(Long.class);

Root<Entity> root = query.from(Entity.class);

//Selecting the count
query.select(cb.count(root));

//Create your search criteria
Criteria criteria = ...

//Adding search criteria   
query.where(criteria);

Long count = getEntityManager().createQuery(query).getSingleResult();

On the other hand, if you want to use JP-QL, the following code should do the trick:

//Add the where condition to the end of the query
Query query = getEntityManager().createQuery("select count(*) from Entity entity where...")
Long count = query.getSingleResult();