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614. Second Degree Follower

Solution: Subquery

Select the followees and the count of followers only if the followee is also a follower in the table.
We can count the number of followers by grouping by followee.

SELECT followee AS follower, COUNT(1) AS num
FROM Follow
WHERE followee IN (
  SELECT follower
  FROM Follow
)
GROUP BY 1
ORDER BY 1;