9/12 每日一題(MYSQL 列出花朵各組的數量)

 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}

# Write your MySQL query statement below

SELECT user_id ,COUNT(follower_id ) AS followers_count

FROM Followers

GROUP BY user_id

ORDER BY user_id