Projděte seznam sloupců tabulky a použijte na ně dotaz

declare
  l_tab_name varchar2(32) := 'MY_TABLE';
begin
  for c1 in (select t.column_name from user_tab_columns t where t.table_name = l_tab_name)
    loop
      execute immediate 'select distinct ' || c1.column_name || ' from ' || l_tab_name;
    end loop;
end;