FindFollowersCount.sql

-- Table: Followers

-- +-------------+------+
-- | Column Name | Type |
-- +-------------+------+
-- | user_id     | int  |
-- | follower_id | int  |
-- +-------------+------+
-- (user_id, follower_id) is the primary key (combination of columns with unique values) for this table.
-- This table contains the IDs of a user and a follower in a social media app where the follower follows the user.
 

-- Write a solution that will, for each user, return the number of followers.

-- Return the result table ordered by user_id in ascending order.

-- The result format is in the following example.
  
-- Example 1:

-- Input: 
-- Followers table:
-- +---------+-------------+
-- | user_id | follower_id |
-- +---------+-------------+
-- | 0       | 1           |
-- | 1       | 0           |
-- | 2       | 0           |
-- | 2       | 1           |
-- +---------+-------------+
-- Output: 
-- +---------+----------------+
-- | user_id | followers_count|
-- +---------+----------------+
-- | 0       | 1              |
-- | 1       | 1              |
-- | 2       | 2              |
-- +---------+----------------+
-- Explanation: 
-- The followers of 0 are {1}
-- The followers of 1 are {0}
-- The followers of 2 are {0,1}

-- Write your PostgreSQL query statement below
-- Solution

select user_id,
       count(follower_id) as followers_count
from Followers group by user_id order by user_id asc;