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leetcode/problem/article-views-i.html

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+<p>Table: <code>Views</code></p>
+
+<pre>
++---------------+---------+
+| 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.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Write an SQL query to find all the authors that viewed at least one of their own articles.</p>
+
+<p>Return the result table sorted by <code>id</code> in ascending order.</p>
+
+<p>The query result format is in the following example.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+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 |
++------------+-----------+-----------+------------+
+<strong>Output:</strong> 
++------+
+| id   |
++------+
+| 4    |
+| 7    |
++------+
+</pre>