如果您正在查找user_connections表定义的所有待处理请求，那么您需要对该表与其自身进行左外连接，如下所示：

架构 (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);

查询#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;