leetcodeSQL：614. Secondary followers

Ideas ：
use in distinguish

sql

# Write your MySQL query statement below
select f1.followee follower, count(f1.followee) num
from follow f1
#  Extract at follower Medium followee that will do 
where f1.followee in
(
    select follower 
    from follow
)
group by f1.followee
order by f1.followee