LeetCode MySQL-1729. Find Followers Count

    AllenLi
    LeetCode MySQL

    Write an SQL query that will, for each user, return the number of followers.
    Return the result table ordered by user_id.

    寫一個 SQL Query來返回每一個使用者的關注數量。
依照 user_id 排序結果。

    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.

    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:
    1. 選擇標題 user_id
    2. 選擇 follower_id 的次數為 followers_count
    3. 以 user_id 為群組
    4. 以 user_id 為排序

    Code1:

    SELECT user_id,
	COUNT(follower_id) AS followers_count
FROM Followers
GROUP BY user_id
ORDER BY user_id;

    newFollower

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