https://nifannn.github.io/2017/10/27/SQL-Notes-Leetcode-614-Second-Degree-Follower/
select followee f1.follower, count(f1.follower) num from follow f1
join follow f2 on f1.followee = f2.follower
group by f1.followee
order by f1.followee;
原文：https://www.cnblogs.com/ffeng0312/p/9835418.html