code: public String getNullValueSelect()
      {
          String ret = null;
          if (!this.types.isEmpty())  {
              ret = Context.getDbType().getNullValueSelect(this.types.iterator().next());
          }
          return ret;
      }
docs: Returns for the first found column type in {@link #types} the related
  SQL select statement for null values.
  
  @return null value select statement
  @see #types