+ Returns the SQL order syntax based on the order parameter passed
+ to the request, and the valid columns.. i.e. 'title ASC' or 'date_created DESC'.
+
+ $sql .= $self->order($r);
+
+ If the order column is not a column of this table,
+ or an order argument is not passed, then the return value is undefined.
+
+ Note: the returned value does not start with a space.
+