CREATE TABLE `ask_interest`
(
        `question_id` INTEGER  NOT NULL,
        `user_id` INTEGER  NOT NULL,
        `created_at` DATETIME,
        PRIMARY KEY (`question_id`,`user_id`)
)Type=MyISAM; 

mysql> describe ask_interest;
+-------------+----------+------+-----+---------+-------+
| Field       | Type     | Null | Key | Default | Extra |
+-------------+----------+------+-----+---------+-------+
| question_id | int(11)  | NO   | PRI |         |       |
| user_id     | int(11)  | NO   | PRI |         |       |
| created_at  | datetime | YES  |     | NULL    |       |
+-------------+----------+------+-----+---------+-------+
3 rows in set (0.11 sec)

mysql>