[SQL50] 1729. Find Followers Count

난이도: EASY

 

문제 링크: https://leetcode.com/problems/find-followers-count/?envType=study-plan-v2&envId=top-sql-50

문제

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}

user_id 별로 묶어서 / 팔로워 수 세고 / user_id 기준 오름차순으로 정렬

---

 

나의 최종 제출 답안:

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