select columns to string

set serveroutput on
DECLARE 
l_next_id integer;
l_total integer;
l_dml varchar2(1000);
l_string varchar2(2000);
l_column varchar2(100);
begin

l_string :='';

select count(1)  into l_total from all_tab_cols where OWNER='' AND table_name='';

for i in 1..l_total
loop
--dbms_output.put_line(i);
select COLUMN_NAME into l_column from all_tab_cols where OWNER='' AND table_name='' and COLUMN_ID=i;
IF l_string is null
THEN
l_string := l_column;
ELSE
l_string := l_string ||','||l_column;
end if;
end loop;
dbms_output.put_line(l_string);
end;