1148. Article Views I - Easy
문제
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Table: Views
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| article_id | int |
| author_id | int |
| viewer_id | int |
| view_date | date |
+---------------+---------+
There is no primary key for this table, it 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 an SQL query 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 query result format is in the following example.
입출력 예
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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 |
+------+
풀이
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SELECT DISTINCT v.author_id as id
FROM Views as v
WHERE v.author_id = v.viewer_id
ORDER BY v.author_id ASC;