Oracle nls length semantics

Questo breve script plsql converte tutte le colonne VARCHAR2 e CHAR da BYTE a CHAR:

begin
 for c1 in (select c.owner,c.table_name,c.column_name,c.data_type,c.data_length
              from dba_tab_columns c inner join dba_tables t on t.owner=c.owner and t.table_name=c.table_name
             where c.data_type in ('VARCHAR2','CHAR') and c.char_used='B' and c.owner in ('TEST'))
 loop
   begin
   execute immediate 'alter table ' || c1.owner || '.' || c1.table_name || ' modify ' || c1.column_name || ' ' || c1.data_type || '(' || c1.data_length || ' char)';
   exception
     when others
         then
             null;
   end;
 end loop;
end;
/