Jika anda mencari semua permintaan belum selesai yang ditakrifkan oleh jadual user_connections, maka anda perlu melakukan sambung luar kiri antara jadual itu dan jadual itu sendiri, seperti ini:

Seni bina (MySQL v5.7)

create table user_connections (
  user_from int,
  user_to int,
  primary key(user_from, user_to)
  );
  
  insert into user_connections(user_from, user_to) values(1, 2);
  insert into user_connections(user_from, user_to) values(2, 1);
  insert into user_connections(user_from, user_to) values(67, 1);
  insert into user_connections(user_from, user_to) values(68, 1);
  insert into user_connections(user_from, user_to) values(69, 1);
  insert into user_connections(user_from, user_to) values(70, 1);

Pertanyaan #1

select uc1.user_from, uc1.user_to from
user_connections uc1 left join
user_connections uc2 on uc2.user_from = uc1.user_to and uc2.user_to = uc1.user_from
where uc2.user_from is null;