Leetcode: Second Degree Follower

Second Degree Follower

Similar Problems:

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         |

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.

Followee would not follow himself/herself in all cases.
Please display the result in follower’s alphabet order.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

## Blog link: https://code.dennyzhang.com/second-degree-follower
## Explain the business logic
##   A follows B. Then A is follwer, B is followee
## What are second degree followers?
##   A follows B, and B follows C. 
##   Then A is the second degree followers of C
select f1.follower, count(distinct f2.follower) as num
from follow as f1 inner join follow as f2
on f1.follower = f2.followee
group by f1.follower

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