Nhibernate count distinct (based on multiple columns)

问题

Basically, i have been trying to do this (count distinct based on two columns):

select count(distinct(checksum(TableA.PropertyA, TableB.PropertyB))) 
from TableA 
left outer join TableB
on TableA.TableBId = TableB.Id 
where PropertyA like '%123%'

Been googling on how to do this but with no luck. Tried this, but never actually worked. This does not count distinctly based on the two properties from two tables:

var queryOver = c.QueryOver<TableA>();
TableB tableBAlias = null;
TableA tableAAlias = null;
ProjectionList projections = Projections.ProjectionList();

queryOver.AndRestrictionOn(x => x.PropertyA).IsLike("%123%");
projections.Add(Projections.CountDistinct(() => tableAAlias.PropertyA));

queryOver.JoinAlias(x => x.TableB , () => tableBAlias, JoinType.LeftOuterJoin);
projections.Add(Projections.CountDistinct(() => tableBAlias.PropertyB));

queryOver.Select(projections);
queryOver.UnderlyingCriteria.SetProjection(projections);
return queryOver.TransformUsing(Transformers.DistinctRootEntity).RowCount();

回答1:

Okay this is going to take a few steps, so bear with me. I'm assuming SQL server here, but the instructions should work for any dialect that supports checksum¹:

Create a custom dialect that supports the checksum function:

public class MyCustomDialect : MsSql2008Dialect
{
    public MyCustomDialect()
    {
        RegisterFunction("checksum", new SQLFunctionTemplate(NHibernateUtil.Int32, "checksum(?1, ?2)"));
    }
}

Update your configuration to use the custom dialect (you can do this either in your configuration XML file or with code. See this answer for more information). Here's how I did it inside of my existing configuration code:
```
configuration
    .Configure(@"hibernate.cfg.xml")
    .DataBaseIntegration(
        db => db.Dialect<MyCustomDialect>());
```

Create a custom projection that calls checksum. This step is optional-- you can call Projections.SqlFunction directly if you'd like, but I think refactoring it into a separate function is cleaner:

public static class MyProjections 
{
    public static IProjection Checksum(params IProjection[] projections)
    {
        return Projections.SqlFunction("checksum", NHibernateUtil.Int32, projections);   
    }
}

Write your QueryOver query and call the custom projection:

int count = session.QueryOver<TableA>(() => tableAAlias)
    .Where(p => p.PropertyA.IsLike("%123%"))
    .Left.JoinQueryOver(p => p.TableB, () => tableBAlias)
    .Select(
        Projections.Count(
            Projections.Distinct(
            MyProjections.Checksum(
                Projections.Property(() => tableAAlias.PropertyA),
                Projections.Property(() => tableBAlias.PropertyB)))))
    .SingleOrDefault<int>();

This should generate SQL that looks like what you're after:

SELECT count(distinct checksum(this_.PropertyA, tableba1_.PropertyB)) as y0_
FROM   [TableA] this_
    left outer join [TableB] tableba1_
    on this_.TableBId = tableba1_.Id
WHERE  this_.PropertyA like '%123%' /* @p0 */

¹Still trying to figure out if there's a way to map a function without manually specifying the number of arguments

来源：https://stackoverflow.com/questions/21725327/nhibernate-count-distinct-based-on-multiple-columns

标签

sql

nhibernate

queryover

nhibernate-criteria

nhibernate-projections