30 Days Of LeetCode Database Day 22 - 1148. Article Views I

1Table: Views
2 
3+---------------+---------+
4| Column Name   | Type    |
5+---------------+---------+
6| article_id    | int     |
7| author_id     | int     |
8| viewer_id     | int     |
9| view_date     | date    |
10+---------------+---------+
11There is no primary key (column with unique values) for this table, the table may have duplicate rows.
12Each row of this table indicates that some viewer viewed an article (written by some author) on some date. 
13Note that equal author_id and viewer_id indicate the same person.

1Example 1:
2 
3Input: 
4Views table:
5+------------+-----------+-----------+------------+
6| article_id | author_id | viewer_id | view_date  |
7+------------+-----------+-----------+------------+
8| 1          | 3         | 5         | 2019-08-01 |
9| 1          | 3         | 6         | 2019-08-02 |
10| 2          | 7         | 7         | 2019-08-01 |
11| 2          | 7         | 6         | 2019-08-02 |
12| 4          | 7         | 1         | 2019-07-22 |
13| 3          | 4         | 4         | 2019-07-21 |
14| 3          | 4         | 4         | 2019-07-21 |
15+------------+-----------+-----------+------------+
16Output: 
17+------+
18| id   |
19+------+
20| 4    |
21| 7    |
22+------+
23 

1**SQL**
2 
3SELECT DISTINCT author_id AS 'id' FROM Views
4WHERE author_id = viewer_id ORDER BY author_id;