Hibernate: how to make EXISTS query? (not subquery)

Question

For example:

EXISTS ( SELECT * FROM [table] WHERE ... )

How to make such query using Hibernate?

Answer 1

HQL doesn't allow to use exists statement. UPD Starting from Hibernate 5 it supports it as a predicate in WHERE

You can use several approaches:

1. For Hibrnate 5: you can use subquery with the same table boolean exists = session.createQuery("select 1 from PersistentEntity where exists (select 1 from PersistentEntity p where ...)").uniqueResult() != null;. Thank to author bellow.
2. count(*) > 0 but this is bad for performance Avoid Using COUNT() in SQL When You Could Use EXISTS()
3. Use boolean exists = session.createQuery("from PersistentEntity where ...").setMaxResults(1).uniqueResult() != null; but this will force Hibernate to load all fields and make hydration to object even if you need just to check for a null.
4. Use session.get(PersistentEntity.class, id) != null and this will work faster if you enabled second level cache but it will be a problem if you need more criteries than just id.
5. You can use the following method: getSession().createQuery("select 1 from PersistentEntity where ...").uniqueResult() != null)

Answer 2

If your goal is inspect some set on emptiness, you may use simple HQL query:

boolean exists = (Long) session.createQuery("select count(*) from PersistentEntity where ...").uniqueResult() > 0