create table
friendship(
user bigint,
friend bigint,
primary key(user, friend),
key(friend, user),
constraint `fk_user` foreign key (user) references user(id),
constraint `fk_friend` foreign key (friend) references user(id)
);
insert into friendship (user, friend) values (1,2);
如果用户2拒绝该请求，
delete from friendship where user = 1 and friend = 2;
如果用户2接受：
insert into friendship (user, friend) values (2,1);
然后，可以通过这种方式找到友谊：
select f1.*
from friendship f1
inner join friendship f2 on f1.user = f2.friend and f1.friend = f2.user;
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