每日必備

#添加外鍵 先保證刪除了 再添加
ALTER TABLE `student_info` DROP FOREIGN KEY `courseid`;

ALTER TABLE `student_info` ADD CONSTRAINT `courseid` FOREIGN KEY (`course_id`) 
REFERENCES `course_info` (`course_id`) ON DELETE RESTRICT ON UPDATE RESTRICT;

LOCK TABLES `student_info` WRITE;
ulock tables；