select
case when exists (select true from table_name where table_column=?)
then 'true'
else 'false'
end;
Mas seria melhor apenas retornar boolean em vez de string:
select exists (select true from table_name where table_column=?);
select
case when exists (select true from table_name where table_column=?)
then 'true'
else 'false'
end;
select exists (select true from table_name where table_column=?);