How to select only a few columns in my NHibernate query?

Easy to do with LINQ (assuming you're using NHibernate 3.0 or later):

var products = from p in Session.Query<Product>()
               where // ...some query (snip)
               select new
               {
                   Name = p.ProductName,
                   Description = p.ShortDesc,
                   Price = p.Price,
                   Units = p.Quantity
               };

Also, if you're using HQL, you can just select the columns you need similar to using T-SQL, but use a Transformer to get a strongly typed object back:

First create a class with your narrowed down columns:

public class ProductReport
{
    public string Name { get; set; }
    public string Description { get; set; }
    public decimal Price { get; set; }
    public int Units { get; set; }
}

Then your query:

string hql = "select p.ProductName as Name, p.ShortDesc as Description ...(snip) " +
             "from Product p " +
             "where ...some query (snip)";

IQuery query = Session.CreateQuery(hql)
    .SetResultTransformer(Transformers.AliasToBean<ProductReport>());

IList<ProductReport> products = query.List<ProductReport>();

Just sure make the aliases in your query (as Name, as Description etc.) match the property names in your class.

In addition to the example Tim gave you can do something like this:

IList selection =
    session.QueryOver<Cat>()
        .Select(
            c => c.Name,
            c => c.Age)
        .List<object[]>();

Above example was taken from: http://nhforge.org/blogs/nhibernate/archive/2009/12/17/queryover-in-nh-3-0.aspx