[LeetCode] 1729. Find Followers Count - SQL

링크: https://leetcode.com/problems/find-followers-count/

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}

---

문제풀이

목표: 각 사용자에 대해 팔로워의 수를 나타내는 솔루션 작성

• 결과는 user_id 를 기준으로 오름차순 정렬

최종코드

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

• follower_id → 몇 명이 팔로우 하고 있는지로 해석하여 오답이 되지 않도록 주의!