将空值保留在nhibernate排序中

Question

I have to keep null values always last in sorting, irrespect of sorting order(asc or desc).

I used code something like this.

criteria
   .AddOrder(Order.Desc(
          Projections.Cast(
                 NHibernateUtil.Decimal, 
                 Projections.Property("col1"))));

criteria
     .AddOrder(Order.Asc(
           Projections.Cast(
                 NHibernateUtil.Decimal, 
                 Projections.Property("col1"))));

i have read this link :

http://www.techrepublic.com/blog/datacenter/control-null-data-in-oracle-using-the-order-by-clause/121

UPDATE

In this link you can see nulls last for sorting. how can i implement this in nhibernate in above code? any help?

Answer 1

We can use standard SQL Function COALESCE and use it as a projection for ORDER BY

using NHibernate.Dialect.Function;
using NHibernate.Criterion;
...

// sql function definition
var sqlFunction = new SQLFunctionTemplate(NHibernateUtil.String
                                         , "COALESCE(Col1, 'zzzz')");
// create projection
var projection = Projections.SqlFunction(sqlFunction, NHibernateUtil.String);

// order by projection
criteria.AddOrder(new Order(projection, false));

And it depends on your needs how will be NULL substituted ('zzzzz' or '000' or ..) to move it at the end or at the beginning. So if the order is ascending projection will use high chars if descending low chars. While there is some overhead, it will do that job