find-followers-count.sql

/*

Find Followers Count Problem

Description
LeetCode Problem 1729.

Table: Followers

+-------------+------+
| Column Name | Type |
+-------------+------+
| user_id     | int  |
| follower_id | int  |
+-------------+------+
(user_id, follower_id) is the primary key 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 an SQL query that will, for each user, return the number of followers.

Return the result table ordered by user_id.

The query result format is in the following example:

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

*/

# V0

# V1
# https://circlecoder.com/find-followers-count/
select user_id, count(distinct follower_id) followers_count
from followers
group by user_id
order by user_id

# V2
# Time:  O(nlogn)
# Space: O(n)
SELECT user_id, 
       Count(follower_id) AS followers_count 
FROM   followers 
GROUP  BY user_id 
ORDER  BY user_id;