text: /* 614. Second Degree Follower
  In facebook, there is a follow table with two columns: followee, follower.
  
  Please write a sql query to get the amount of each follower’s follower if he/she has one.
  
  For example:
  
  +-------------+------------+
  | followee    | follower   |
  +-------------+------------+
  |     A       |     B      |
  |     B       |     C      |
  |     B       |     D      |
  |     D       |     E      |
  +-------------+------------+
  should output:
  +-------------+------------+
  | follower    | num        |
  +-------------+------------+
  |     B       |  2         |
  |     D       |  1         |
  +-------------+------------+
  Explaination:
  Both B and D exist in the follower list, when as a followee, B's follower is C and D, and D's follower is E. A does not exist in follower list.
  
  Note:
  Followee would not follow himself/herself in all cases.
  Please display the result in follower's alphabet order.
  */
  
  -- Solution 
  select 
      followee as folower,
      count(distinct follower) as num
  from follow
  where followee in (select distinct follower from follow)
  group by 1
  order by 1
  
  