第一，你只是缺少一个别名为你的子查询如下-

Select a2,b1
from (
    SELECT a.uid_1,a.uid_2 as a2,b.uid_1 as b1,b.uid_2
    from friend a join
    friend b
    on a.uid_1=b.uid_2
) A -- added A as a Alias

但是第二点，不确定您要对连接做什么，因为您的查询实现了连接条件，这与下面的查询是等价的-

SELECT * 
FROM friend 
WHERE uid_1 = uid_2

您需要子查询的表名别名，例如：select。。。从（子查询）t
然后可以使用完全限定名引用子查询内容

Select T.a2, T.b1
    from (SELECT a.uid_1,a.uid_2 as a2, b.uid_1 as b1, b.uid_2 b2
          from friend a join
               friend b
               on a.uid_1=b.uid_2
         ) T

你刚才不见了

Select a2,b1
from (SELECT a.uid_1,a.uid_2 as a2,b.uid_1 as b1,b.uid_2
  from friend a join
       friend b
       on a.uid_1=b.uid_2
 ) as temp -- here it is