article-views-i.sql

/*
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.

 

Example 1:

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    |
+------+
*/
SELECT DISTINCT B.author_id id 
FROM
    (
        SELECT article_id
            ,SUM(CASE WHEN author_id = viewer_id THEN 1 ELSE 0 END) AS cnt_authors
        FROM Views
        GROUP BY article_id
        HAVING SUM(CASE WHEN author_id = viewer_id THEN 1 ELSE 0 END) <> 0
    ) A
LEFT JOIN Views B
ON A.article_id = B.article_id
WHERE B.author_id = B.viewer_id 
ORDER BY id ASC
;