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MariaDB [market]> alter table user_order1 add foreign key(u_id) references userprofile(id) on delete cascade on update cascade; # 使用alter修改user_order1数据表，为user_order1数据表添加外键约束
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [market]> show create table user_order1; # 查看user_order1数据表的创建信息，已经添加了外键约束
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user_order1 | CREATE TABLE `user_order1` (
`o_id` int(11) NOT NULL AUTO_INCREMENT,
`u_id` int(11) DEFAULT '0',
`username` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
`money` int(11) DEFAULT NULL,
PRIMARY KEY (`o_id`),
KEY `u_id` (`u_id`),
CONSTRAINT `user_order1_ibfk_1` FOREIGN KEY (`u_id`) REFERENCES `userprofile` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
3.删除外键
语法
alter table 数据表名称 drop foreign key 约束（外键）名称
例子：
MariaDB [market]> show create table user_order1; # 查看user_order1数据表的创建信息，包含外键约束
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user_order1 | CREATE TABLE `user_order1` (
`o_id` int(11) NOT NULL AUTO_INCREMENT,
`u_id` int(11) DEFAULT '0',
`username` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
`money` int(11) DEFAULT NULL,
PRIMARY KEY (`o_id`),
KEY `u_id` (`u_id`),
CONSTRAINT `user_order1_ibfk_1` FOREIGN KEY (`u_id`) REFERENCES `userprofile` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec) （编辑：核心网）
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