SQL Interview Questions – All the Pairs With the Maximum Number of Common Followers

Problem Description –

Write an SQL query to find all the pairs of users with the maximum number of common followers. In other words, if the maximum number of common followers between any two users is maxCommon, then you have to return all pairs of users that have maxCommon common followers.

The result table should contain the pairs user1_id and user2_id where user1_id < user2_id.

Return the result table in any order.

The query result format is in the following example.

Problem Link – Common Followers

Difficulty Level – Medium

Solution –

select 
    user1_id, 
    user2_id
from
(select 
    r1.user_id as user1_id, 
    r2.user_id as user2_id, 
    dense_rank()over(order by count(r1.follower_id) desc) as rk
from Relations r1, Relations r2
where r1.user_id < r2.user_id and r1.follower_id = r2.follower_id 
group by r1.user_id, r2.user_id) temp
where rk = 1