1148. Article Views I

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

Description

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, sorted in ascending order by their id.

The query result format is in the following example:

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 |
+------------+-----------+-----------+------------+
Result table:
+------+
| id   |
+------+
| 4    |
| 7    |
+------+

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