When to use Hibernate projections?

Question

I am a little confused about Hibernate\'s projections and criteria. When to use projections and when to use criteria?

Answer 1

Projections are used to execute aggregate operations and to get single column query,with Restrictions we can access a ROW but with PROJECTIONS we can access whole COLUMN

EX -

public static void main(String[] args) {
    SessionFactory factory = new Configuration().configure().addAnnotatedClass(Student.class).buildSessionFactory();
    Session session = factory.getCurrentSession();
    try {
        session.beginTransaction();
        Criteria c = session.createCriteria(Student.class);
        Projection p = Projections.property("lastName");
        List students = c.setProjection(p).list();
        for(String s:students)
            System.out.println(s);
        session.getTransaction().commit();
        session.close();
    } finally {
        factory.close();
    }
}

In the above example i used projection call to ADD a projection property "name" to the criteria. It returns winchester winchester winchester winchester , which is the lastName COLUMN in the table.

Output =

Hibernate: select this_.last_name as y0_ from student this_ winchester winchester winchester winchester

Note - We can add only one projection, if we add more than 1 projection previous one will be overridden. if you want to add more than one projection you will nedd ProjectionList class

Orignal Table -