Oracle 18c

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create table countries (cod char(2));
insert into countries
select 'GB' from dual union all
select 'CH' from dual;
2 rows affected

create table strings(s varchar(25));
insert into strings values ('GB')
1 rows affected

insert into strings values ('CH')
1 rows affected

insert into strings values ('GB,CH')
1 rows affected

insert into strings values ('CH,GB')
1 rows affected

insert into strings values ('FR')
1 rows affected

insert into strings values ('ES')
1 rows affected

insert into strings values ('CH')
1 rows affected

insert into strings values ('ES,CH')
1 rows affected

insert into strings values ('GB,ES')
1 rows affected

with cte as
(select distinct
s,regexp_substr(s, '[^,]+',1, level) code from strings
    
 connect by regexp_substr(s, '[^,]+', 1, level) is not null
/*and prior s = s*/
)
select 
s string,min(case when exists
  (select * from countries
   where cod = code) then 'yes'
   else 'no'end) all_found
from cte
group by s
order by s;




STRING
ALL_FOUND


CH
yes

CH,GB
yes

ES
no

ES,CH
no

FR
no

GB
yes

GB,CH
yes

GB,ES
no