Distinct query in Criteria NHibernate - distinct

Question

I need the corresponding query in Criteria language to this one (to retrieve all categories from my table but to distinct them):
SELECT DISTINCT categoryName
FROM Category
WHERE CategoryID IN (
SELECT CategoryID
FROM FoodCategory
)
ORDER BY categoryName
I have table FoodCategory table
id | FoodID | CategoryID
--------|---------------|------------
| |
| |
| |
Actually CategoryID is a foreign key that is pointing to this table here. This is table for Category:
CategoryID | categoryName | otherField
---------------|------------------|------------
| |
| |
| |
And this is table for Food:
FoodID | FoodName | otherField
---------------|------------------|------------
| |
| |
| |

Answer 1

Something like that should do the trick :
public List<String> retrieveFoodCategoryNames() {
CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<String> criteriaQuery = builder.createQuery(String.class);
Root<FoodCategory> root = criteriaQuery.from(FoodCategory.class);
// by default an inner join so it will only get the categories which have their id in the FoodCategory table
Join<FoodCategory, Category> joinCategory = root.join(FoodCategory_.category);
Fetch<FoodCategory, Category> fetchCategory = root.fetch(FoodCategory_.category);
Path<String> categoryNamePath = fetchCategory.get(Category_.categoryName);
criteriaQuery.select(categoryNamePath).distinct(true);
criteriaQuery.orderBy(builder.asc(categoryNamePath));
return entityManager.createQuery(criteriaQuery).getResultList();
}
This is not the exact same SQL request because you used a subquery where I'm using a join but it seemed more suited to this particular case. The subquery syntax is a bit more complex and I will not try to write it without compiling! ^^
If something is unclear let me know :-)