1.什么是外键约束

外键约束(foreign key)就是表与表之间的某种约定的关系，由于这种关系的存在，我们能够让表与表之间的数据，更加的完整，关连性更强。

关于数据表的完整性和关连性，可以举个例子

有二张表，一张是用户表，一张是订单表：

1.如果我删除了用户表里的用户，那么订单表里面跟这个用户有关的数据，就成了无头数据了，不完整了。
2.如果我在订单表里面，随便插入了一条数据，这个订单在用户表里面，没有与之对应的用户。这样数据也不完整了。

如果有外键的话，就方便多了，可以不让用户删除数据，或者删除用户的话，通过外键同样删除订单表里面的数据，这样也能让数据完整。

通过外键约束，每次插入或更新数据表时，都会检查数据的完整性。

2.创建外键约束

2.1 方法一：通过create table创建外键

语法：

create table 数据表名称(
...,
[CONSTRAINT [约束名称]] FOREIGN KEY [外键字段] 
    REFERENCES [外键表名](外键字段，外键字段2…..)
    [ON DELETE CASCADE ]
    [ON UPDATE CASCADE  ]
)

参数的解释：

RESTRICT: 拒绝对父表的删除或更新操作。
CASCADE: 从父表删除或更新且自动删除或更新子表中匹配的行。ON DELETE CASCADE和ON UPDATE CASCADE都可用

注意：on update cascade是级联更新的意思，on delete cascade是级联删除的意思，意思就是说当你更新或删除主键表，那外键表也会跟随一起更新或删除。

精简化后的语法：

foreign key 当前表的字段 references 外部表名 (关联的字段） type=innodb 

2.1.1 插入测试数据

例子：我们创建一个数据库，包含用户信息表和订单表

MariaDB [book]> create database market;             # 创建market数据库
Query OK, 1 row affected (0.00 sec)

MariaDB [book]> use market;                         # 使用market数据库
Database changed

MariaDB [market]> create table userprofile(id int(11) not null auto_increment, name varchar(50) not null default '', sex int(1) not null default '0', primary key(id))ENGINE=innodb;    # 创建userprofile数据表，指定使用innodb引擎
Query OK, 0 rows affected (0.07 sec)

MariaDB [market]> create table user_order(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), foreign key order_f_key(u_id) references userprofile(id) on delete cascade on update cascade);            # 创建user_order数据表，同时为user_order表的u_id字段做外键约束，绑定userprofile表的id字段
Query OK, 0 rows affected (0.04 sec)

MariaDB [market]> insert into userprofile(name,sex)values('HA',1),('LB',2),('HPC',1);       # 向userprofile数据表插入三条记录
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [market]> select * from userprofile;        # 查询userprofile数据表的所有记录
+----+------+-----+
| id | name | sex |
+----+------+-----+
|  1 | HA   |   1 |
|  2 | LB   |   2 |
|  3 | HPC  |   1 |
+----+------+-----+
3 rows in set (0.00 sec)

MariaDB [market]> insert into user_order(u_id,username,money)values(1,'HA',234),(2,'LB',146),(3,'HPC',256);                     # 向user_order数据表插入三条记录
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [market]> select * from user_order;         # 查询user_order数据表的所有记录
+------+------+----------+-------+
| o_id | u_id | username | money |
+------+------+----------+-------+
|    1 |    1 | HA       |   234 |
|    2 |    2 | LB       |   146 |
|    3 |    3 | HPC      |   256 |
+------+------+----------+-------+
3 rows in set (0.00 sec)

MariaDB [market]> select id,name,sex,money,o_id from userprofile,user_order where id=u_id;      # 联表查询
+----+------+-----+-------+------+
| id | name | sex | money | o_id |
+----+------+-----+-------+------+
|  1 | HA   |   1 |   234 |    1 |
|  2 | LB   |   2 |   146 |    2 |
|  3 | HPC  |   1 |   256 |    3 |
+----+------+-----+-------+------+
3 rows in set (0.03 sec)

2.1.2 测试级联删除

MariaDB [market]> delete from userprofile where id=1;       # 删除user表中id为1的数据
Query OK, 1 row affected (0.01 sec)

MariaDB [market]> select id,name,sex,money,o_id from userprofile,user_order where id=u_id;
+----+------+-----+-------+------+
| id | name | sex | money | o_id |
+----+------+-----+-------+------+
|  2 | LB   |   2 |   146 |    2 |
|  3 | HPC  |   1 |   256 |    3 |
+----+------+-----+-------+------+
2 rows in set (0.00 sec)

MariaDB [market]> select * from user_order;                 # 查看order表的数据
+------+------+----------+-------+
| o_id | u_id | username | money |
+------+------+----------+-------+
|    2 |    2 | LB       |   146 |
|    3 |    3 | HPC      |   256 |
+------+------+----------+-------+
3 rows in set (0.00 sec)

2.1.3 测试级联更新

更新数据之前的状态

MariaDB [market]> select * from userprofile;                # 查看userprofile表的数据
+----+------+-----+
| id | name | sex |
+----+------+-----+
|  2 | LB   |   2 |
|  3 | HPC  |   1 |
+----+------+-----+
3 rows in set (0.00 sec)

MariaDB [market]> select * from user_order;                 # 查看order表的数据
+------+------+----------+-------+
| o_id | u_id | username | money |
+------+------+----------+-------+
|    2 |    2 | LB       |   146 |
|    3 |    3 | HPC      |   256 |
+------+------+----------+-------+
3 rows in set (0.00 sec)

更新数据

MariaDB [market]> update userprofile set id=6 where id=2;   # 把userprofile数据表中id为2的用户改为id为6
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

更新数据后的状态

MariaDB [market]> select id,name,sex,money,o_id from userprofile,user_order where id=u_id;      # 联表查询，可以看出表中已经没有id为2的用户了
+----+------+-----+-------+------+
| id | name | sex | money | o_id |
+----+------+-----+-------+------+
|  6 | LB   |   2 |   146 |    2 |
|  3 | HPC  |   1 |   256 |    3 |
+----+------+-----+-------+------+
2 rows in set (0.00 sec)

MariaDB [market]> select * from userprofile;                # 查看userprofile表的数据，id只剩下3和6
+----+------+-----+
| id | name | sex |
+----+------+-----+
|  3 | HPC  |   1 |
|  6 | LB   |   2 |
+----+------+-----+
2 rows in set (0.00 sec)

MariaDB [market]> select * from user_order;                 # 查看user_order表的数据,u_id也改为6
+------+------+----------+-------+
| o_id | u_id | username | money |
+------+------+----------+-------+
|    2 |    6 | LB       |   146 |
|    3 |    3 | HPC      |   256 |
+------+------+----------+-------+
2 rows in set (0.00 sec)

2.1.4 测试数据完整性

MariaDB [market]> insert into user_order(u_id,username,money)values(5,"XJ",345);        # 单独向user_order数据表中插入数据，插入数据失败
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`market`.`user_order`, CONSTRAINT `user_order_ibfk_1` FOREIGN KEY (`u_id`) REFERENCES `userprofile` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)

在上面的例子中，user_order表的外键约束，user_order表受userprofile表的约束

在user_order里面插入一条数据u_id为5用户，在userprofile表里面根本没有，所以插入数据失败

先向userprofile表中插入记录，再向user_order表中插入记录就可以了

MariaDB [market]> insert into userprofile values(5,"XJ",1);         # 先向userprofile数据表中插入id为5的记录，插入数据成功
Query OK, 1 row affected (0.01 sec)

MariaDB [market]> insert into user_order(u_id,username,money) values(5,"XJ",345);       # 再向user_order数据表中插入数据,成功
Query OK, 1 row affected (0.00 sec)

MariaDB [market]> select * from userprofile;                # 查询userprofile数据表中的所有记录
+----+------+-----+
| id | name | sex |
+----+------+-----+
|  3 | HPC  |   1 |
|  5 | XJ   |   1 |
|  6 | LB   |   2 |
+----+------+-----+
3 rows in set (0.00 sec)

MariaDB [market]> select * from user_order;                 # 查询user_order数据表中的所有记录
+------+------+----------+-------+
| o_id | u_id | username | money |
+------+------+----------+-------+
|    2 |    6 | LB       |   146 |
|    3 |    3 | HPC      |   256 |
|    5 |    5 | XJ       |   345 |
+------+------+----------+-------+
3 rows in set (0.01 sec)

2.2 方法二：通过alter table创建外键和级联更新，级联删除

语法：

alter table 数据表名称 add 
    [constraint [约束名称] ]  foreign key (外键字段,..) references 数据表(参照字段,...) 
    [on update cascade|set null|no action]
    [on delete cascade|set null|no action]
)

例子：

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)

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)

MariaDB [market]> alter table user_order1 drop foreign key user_order1_ibfk_1;          # 为user_order1数据表删除外键约束，外键名称必须与从`show create table user_order1`语句中查到的相同
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，外键约束已经被删除了
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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 (0.00 sec)

4.使用外键约束的条件

要想外键创建成功，必须满足以下4个条件：

1、确保参照的表和字段存在。 
2、组成外键的字段被索引。 
3、必须使用type指定存储引擎为：innodb.
4、外键字段和关联字段，数据类型必须一致。

5.使用外键约束需要的注意事项

1.on delete cascade  on update cascade 添加级联删除和更新：
2.确保参照的表userprofile中id字段存在。
3.确保组成外键的字段u_id被索引
4.必须使用type指定存储引擎为：innodb。
5.外键字段和关联字段，数据类型必须一致。