How to get a list of all classes of a given namespace ?

I use the following query to get a list of all classes : 

select * from %Dictionary.ClassDefinition 
where System=0 and Hidden=0 

It works, but AFAIK it return all classes from all namespaces, including the ones defined in %SYS (usually classes they starts with % like %CSP).

I would like to get only classes of current namespace. I have checked all the columns returned by the query and none allow to filter on a given namespace.

The solution I am looking for does not need to be a query, the query I posted above is called from code.