mysql添加与删除外键

为已经添加好的数据表添加外键: 语法:alter table 表名 add constraint FK_ID foreign key(你的外键字段名) REFERENCES 外表表名(对应的表的主键字段名); 例: alter table tb_active add constraint FK_ID foreign key(user_id) REFERENCES tb_user(id) /* CREATE TABLE `tb_active` (   `id` int(11) NOT NULL AUTO_INCREMENT,   `title` varchar(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,   `content` text CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,   `user_id` int(11) NOT NULL,   PRIMARY KEY (`id`),   KEY `user_id` (`user_id`),   KEY `user_id_2` (`user_id`),   CONSTRAINT `FK_ID` FOREIGN KEY (`user_id`) REFERENCES `tb_user`(`id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 */     删除外键 语法： ALTER TABLE table-name DROP FOREIGN KEY key-id; 例：   ALTER TABLE `tb_active` DROP FOREIGN KEY `FK_ID`