[MySQL] 리트코드 1148. Article Views I

문제

Table: Views

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| article_id    | int     |
| author_id     | int     |
| viewer_id     | int     |
| view_date     | date    |
+---------------+---------+

There is no primary key (column with unique values) for this table, the table may have duplicate rows.
Each row of this table indicates that some viewer viewed an article (written by some author) on some date. 
Note that equal author_id and viewer_id indicate the same person.
 
Write a solution to find all the authors that viewed at least one of their own articles.
Return the result table sorted by id in ascending order.
The result format is in the following example.

 

https://leetcode.com/problems/article-views-i/description/

예시

Input: 
Views table:
+------------+-----------+-----------+------------+
| article_id | author_id | viewer_id | view_date  |
+------------+-----------+-----------+------------+
| 1          | 3         | 5         | 2019-08-01 |
| 1          | 3         | 6         | 2019-08-02 |
| 2          | 7         | 7         | 2019-08-01 |
| 2          | 7         | 6         | 2019-08-02 |
| 4          | 7         | 1         | 2019-07-22 |
| 3          | 4         | 4         | 2019-07-21 |
| 3          | 4         | 4         | 2019-07-21 |
+------------+-----------+-----------+------------+
Output: 
+------+
| id   |
+------+
| 4    |
| 7    |
+------+

문제 풀이

SELECT  DISTINCT author_id AS id
FROM    views
WHERE   author_id = viewer_id
ORDER BY id;

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SQL을 독학하시는 분들에게 도움이 되길 바라며,

혹 더 좋은 방법이 있거나 오류가 있다면 편하게 말씀 부탁드립니다.