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1729. Find Followers Count
Description
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} Solutions
Solution 1: Grouping and Aggregation
We can directly group the Followers table by user_id, and use the COUNT function to count the number of followers for each user.
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# Write your MySQL query statement below SELECT user_id, COUNT(1) AS followers_count FROM Followers GROUP BY 1 ORDER BY 1;