UNION to JPA Query | 易学教程

问题

Is it possible to query "UNION" in JPA and even "Criteria Builder"?

I'm looking for examples, but so far i got no result.

Does anyone have any examples of how to use it?

Or would that be with native sql?

回答1:

SQL supports UNION, but JPA 2.0 JPQL does not. Most unions can be done in terms of joins, but some can not, and some are more difficult to express using joins.

EclipseLink supports UNION.

回答2:

Depending on the case, one could use sub queries, something like:

select e
from Entity e
where e.id in
(
  select e.id
  from Entity2 e2
       join e2.entity e
  where e2.someProperty = 'value'
)
      or e.id in
(
  select e.id
  from Entity3 e3
       join e3.entity e
  where e3.someProperty = 'value2'
)

回答3:

One thing just comes to my mind (searching for the exact same problem):

Perform two different JPA queries on the same entity mapping and simply add the objects of the second result to the list (or set to avoid duplicates) of the first result.

That way you get the same effect as with a UNION, the difference being that you use two SQL statements instead of one. But actually, I would expect that to perform just as good as issueing one UNION statement.

回答4:

write native query (set it true , default its false) - ex.

String findQuery = "select xyz from abc union select abc from def"
@Query(value = findQuery, nativeQuery = true)
//method

回答5:

There is no direct union for JPA, what I did was to build two specifications.

Specification<Task> specification = Specification.where(null);
Specification<Task> specification2 = Specification.where(null;

They belong to a single table but return different values

specification = specification.and((root, criteriaQuery, criteriaBuilder) -> criteriaBuilder.equal(criteriaBuilder.function(MONTH, Integer.class, root.get("deliveryExpirationDate")), month));
        specification2 = specification2.and((root, criteriaQuery, criteriaBuilder) -> criteriaBuilder.lessThan(criteriaBuilder.function(MONTH, Integer.class, root.get("deliveryExpirationDate")), month))
            .and((root, criteriaQuery, criteriaBuilder) -> criteriaBuilder.equal(root.get("enable"), true));

for this example it is a table of tasks that in the first specification I need the tasks of the current month enabled and disabled, and in the second specification I only need the tasks enabled of the previous months.

Specification<Task> specificationFullJoin = Specification.where(specification).or(specification2);

Esto es muy útil para que la lista de tareas devueltas tenga paginación.

taskRepository.findAll(specificationFullJoin, pageable).map(TaskResponse::new); //Here you can continue adding filters, sort or search.

It helps me a lot, I hope it is what they are looking for or that it serves them something.

回答6:

using EntityManager.createNativeQuery(...); It's allow you use UNION

回答7:

You can directly use UNION in your query. The following query returns id of friends from FRIENDSHIP table.

Eg:

TypedQuery<String> queryFriend = em.createQuery("SELECT f.Id FROM Friendship f WHERE f.frStuId = :stuId UNION "
                                 + "SELECT f.frStuId FROM Friendship f WHERE f.FriendId = :stuId", String.class);
queryFriend.setParameter("stuId", stuId);

List<String> futureFriends = queryFriend.getResultList();

来源：https://stackoverflow.com/questions/18958614/union-to-jpa-query

标签

java

jpa

criteria