Sort data in two columns

mysql_sort_columns.sql

/*
 * Creating table:
 */
CREATE TABLE sql_test
( 
    id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY, 
    started_at TIMESTAMP NULL, 
    created_at TIMESTAMP
); 

/*
 * Storing data:
 */
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:30');
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:31');
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:32');
INSERT INTO sql_test (created_at, started_at) VALUES ('2019-07-23 18:33', '2019-08-27 04:07');
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:34');
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:35');
INSERT INTO sql_test (created_at, started_at) VALUES ('2019-07-23 18:33', '2019-06-12 04:07');
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:36');
INSERT INTO sql_test (created_at, started_at) VALUES ('2019-07-23 18:37', '2019-08-24 08:15');
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:38');
INSERT INTO sql_test (created_at) VALUES ('2019-07-23 18:39');

/*
 * Query:
 */
SELECT
  * 
FROM sql_test
ORDER BY IF(started_at IS NULL, created_at, started_at) DESC

/*
 * Result:
 *
 * | id | started_at          | created_at          |
 * | 4  | 2019-08-27 04:07:00 | 2019-07-23 18:33:00 |
 * | 9  | 2019-08-24 08:15:00 | 2019-07-23 18:37:00 |
 * | 11 |                     |	2019-07-23 18:39:00 |
 * | 10 |                     | 2019-07-23 18:38:00 |
 * | 8  |                     |	2019-07-23 18:36:00 |
 * | 6  |                     | 2019-07-23 18:35:00 |
 * | 5  |                     | 2019-07-23 18:34:00 |
 * | 3  |                     | 2019-07-23 18:32:00 |
 * | 2  |                     | 2019-07-23 18:31:00 |
 * | 1  |                     | 2019-07-23 18:30:00 |
 * | 7  | 2019-06-12 04:07:00 | 2019-07-23 18:33:00 |
 */