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1729. Find Followers Count

Level

Easy

Description

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 following relationship 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}

Solution

For each user_id, count the number of entries in Followers. Group the result by user_id and order the result by user_id.

# Write your MySQL query statement below
select user_id, count(*) as followers_count from Followers
    group by user_id
    order by user_id;

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