Second Degree Follower Problem
Description
LeetCode Problem 614.
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:
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+-------------+------------+
| followee | follower |
+-------------+------------+
| A | B |
| B | C |
| B | D |
| D | E |
+-------------+------------+
should output:
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+-------------+------------+
| 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.
MySQL Solution
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select f1.follower, count(distinct f2.follower) as num
from follow f1
join follow f2
on f1.follower = f2.followee
group by f1.follower
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