create table friends(person varchar(5),friend varchar(5));
insert friends
select 'A','B' union all
select 'B','A' union all
select 'B','C' union all
select 'C','B'


select * from
(select f1.person as f1person,f1.friend as f1friend,f2.friend  as f2friend
from friends f1,friends f2
where f1.friend = f2.person) t1
where t1.f1person != t1.f2friend ;