When to use Hibernate projections?

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<String> 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 -

They're not mutually exclusive, you can use both at the same time. Projections are generally used in the context of some Criteria.

To put it simple, Hibernate Projections are used in order to query only a subset of the attributes of an entity or group of entities you're querying with Criteria. You can also use Projections to specify distinct clauses and aggregate functions like max, sum and so on. It's like referring to which data you're fetching. Like modifying the select clause in an SQL query.

Hibernate Criteria are used to define conditions the data has to satisfy in order to be selected. It's like referring to how is the data you're fetching. Like modifiying the from and where clauses of an SQL query.

Note that this how and which is not strictly true, it's just an orientation aimed to aid the OP. You can change which data you're fetching with createCriteria(String associationPath) for instance.

I'd suggest to take a look at this article Hibernate: Criteria Queries in Depth