MariaDB [market]> create table user_order1(o_id int(11) auto_increment,u_id int(11) default "0",username varchar(50),money int(11),primary key(o_id),index(u_id));  # 创建user_order1数据表，创建表时不使用外键约束

Query OK, 0 rows affected (0.11 sec)

MariaDB [market]> show create table user_order1;  # 查看user_order1数据表的创建信息，没有外键约束

+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| 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`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |

+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

1 row in set (0.01 sec)

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)