What is a Projection in NHibernate?

I have a project where I'm using NHibernate to handle bindings to the database. So far I have mostly been using the basics when it comes to queries. Now I'm struggling with a more difficult query, and I notice new parts of NHibernate. In particular I'm curious about SetProjection, which seems to be important when doing queries.

What 开发者_开发百科is a Projection, and how will I typically use it? I am assuming that a projection is a general term when it comes to databases, so you are welcome to give more general answers too..

Projection as Antoine said is transformation. In terms of query it is:

SELECT *PROJECTION* FROM Table

*PROJECTION* is expression for data transformation.

Example:

SELECT * FROM ORDER

The Criteria equivalent would be:

List orders = session.createCriteria(Order.class).list();

No projection here, we take data without transformation. If we want one:

SELECT NAME FROM PRODUCT

Here, the Projection class comes into play. The above query can be rewritten into a Criteria query as:

List products=session.createCriteria(Product.class)
     .setProjection(Projection.property(\"name\"))
     .list();

So we project all rows to single item: name field.

There are other projections: Projection.rowCount() for example (for COUNT(*))

I don't know about NHibernate, but in general, a projection is a transformation of a set into another set. In SQL, it is expressed as a SELECT.