create table letters (
   letter       varchar2(1 char) not null constraint letters_pk primary key,
   occurrences  integer          not null,
   is_vowel     integer          generated always as (
                                    case 
                                       when letter in ('a', 'e', 'i', 'o', 'u') then 
                                          1 
                                       else
                                          0 
                                     end
                                 ) virtual
);

create table letters (
   letter       varchar2(1 char) not null constraint letters_pk primary key,
   occurrences  integer          not null,
   is_vowel     integer          generated always as (
                                    case
   when letter in ('a', 'e', 'i', 'o', 'u') then
                                          1
   else
                                          0
end
                                 ) virtual
);