SQL Schema
Create table If Not Exists Followers(user_id int, follower_id int);
Truncate table Followers;
insert into Followers (user_id, follower_id) values ('0', '1');
insert into Followers (user_id, follower_id) values ('1', '0');
insert into Followers (user_id, follower_id) values ('2', '0');
insert into Followers (user_id, follower_id) values ('2', '1');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.
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}
Solution
select user_id,
count(distinct(follower_id)) as followers_count
from Followers
group by user_id