A PRIMARY KEY can be a multiple-column index. However, you cannot create a multiple-column index using the PRIMARY KEY key attribute in a column specification. Doing so only marks that single column as primary. You must use a separate PRIMARY KEY(index_col_name, ...) clause.AndCREATE TABLE `ask_interest`( `question_id` INTEGER NOT NULL, `user_id` INTEGER NOT NULL, `created_at` DATETIME, PRIMARY KEY (`question_id`,`user_id`), CONSTRAINT `ask_interest_FK_1` FOREIGN KEY (`question_id`) REFERENCES `ask_question` (`id`), INDEX `ask_interest_FI_2` (`user_id`), CONSTRAINT `ask_interest_FK_2` FOREIGN KEY (`user_id`) REFERENCES `ask_user` (`id`))Type=MyISAM;