This one took a while to figure out and it required a few beers. Let's say you have an entity called Product and an entity called ProductOwner and there is a 1 to n relationship from ProductOwner to Product but you can only access Product from ProductOwner.
This may not be the way the database is designed - that would be terrible, but it may be the way your model has been designed. Probably an oversight but in any case, now you're stuck with it.
What do you do if you want find out how many ProductOwners don't actually own any products? It can happen, but because of the design of the model, it's not obvious how to fetch those records.
In SQL, it's easy...
select * from ProductOwner po where po.id not in (select p.id from product)
Using JPA CriteriaBuilder you would do the following
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CriteriaBuilder cb = entityManager.getCriteriaBuilder(); CriteriaQuery<ProductOwner> query = cb.createQuery(ProductOwner.class); Root<ProductOwner> poRoot = query.from(ProductOwner.class); query.select(poRoot); Subquery<Product> subquery = query.subquery(Product.class); Root<Product> subRoot = subquery.from(Product.class); subquery.select(subRoot); Predicate p = cb.equal(subRoot.get(Product_.productOwner),poRoot); subquery.where(p); query.where(cb.not(cb.exists(subquery))); TypedQuery<ProductOwner> typedQuery = entityManager.createQuery(query); List<ProductOwner> result = typedQuery.getResultList();