詳解MariaDB數據庫的外鍵約束

1.什么是外鍵約束

外鍵約束(foreign key)就是表與表之間的某種約定的關系，由於這種關系的存在，我們能夠讓表與表之間的數據，更加的完整，關連性更強。

關於數據表的完整性和關連性，可以舉個例子

有二張表，一張是用戶表，一張是訂單表：

1.如果我刪除了用戶表里的用戶，那么訂單表里面跟這個用戶有關的數據，就成了無頭數據了，不完整了。
2.如果我在訂單表里面，隨便插入了一條數據，這個訂單在用戶表里面，沒有與之對應的用戶。這樣數據也不完整了。

如果有外鍵的話，就方便多了，可以不讓用戶刪除數據，或者刪除用戶的話，通過外鍵同樣刪除訂單表里面的數據，這樣也能讓數據完整。

通過外鍵約束，每次插入或更新數據表時，都會檢查數據的完整性。

2.創建外鍵約束

2.1 方法一：通過create table創建外鍵

語法：

create table 數據表名稱(
...,
[CONSTRAINT [約束名稱]] FOREIGN KEY [外鍵字段] 
    REFERENCES [外鍵表名](外鍵字段，外鍵字段2…..)
    [ON DELETE CASCADE ]
    [ON UPDATE CASCADE  ]
)

參數的解釋：

RESTRICT: 拒絕對父表的刪除或更新操作。
CASCADE: 從父表刪除或更新且自動刪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是級聯更新的意思，on delete cascade是級聯刪除的意思，意思就是說當你更新或刪除主鍵表，那外鍵表也會跟隨一起更新或刪除。

精簡化后的語法：

foreign key 當前表的字段 references 外部表名 (關聯的字段） type=innodb 

2.1.1 插入測試數據

例子：我們創建一個數據庫，包含用戶信息表和訂單表

MariaDB [book]> create database market;				# 創建market數據庫
Query OK, 1 row affected (0.00 sec)

MariaDB [book]> use market;							# 使用market數據庫
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;	# 創建userprofile數據表，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);			# 創建user_order數據表，同時為user_order表的u_id字段做外鍵約束，綁定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數據表插入三條記錄
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [market]> select * from userprofile;		# 查詢userprofile數據表的所有記錄
+----+------+-----+
| 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數據表插入三條記錄
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [market]> select * from user_order;			# 查詢user_order數據表的所有記錄
+------+------+----------+-------+
| 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;		# 聯表查詢
+----+------+-----+-------+------+
| 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 測試級聯刪除

MariaDB [market]> delete from userprofile where id=1;		# 刪除user表中id為1的數據
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表的數據
+------+------+----------+-------+
| o_id | u_id | username | money |
+------+------+----------+-------+
|    2 |    2 | LB       |   146 |
|    3 |    3 | HPC      |   256 |
+------+------+----------+-------+
3 rows in set (0.00 sec)

2.1.3 測試級聯更新

更新數據之前的狀態

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

更新數據

MariaDB [market]> update userprofile set id=6 where id=2;	# 把userprofile數據表中id為2的用戶改為id為6
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

更新數據后的狀態

MariaDB [market]> select id,name,sex,money,o_id from userprofile,user_order where id=u_id;		# 聯表查詢，可以看出表中已經沒有id為2的用戶了
+----+------+-----+-------+------+
| 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表的數據，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表的數據,u_id也改為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 測試數據完整性

MariaDB [market]> insert into user_order(u_id,username,money)values(5,"XJ",345);		# 單獨向user_order數據表中插入數據，插入數據失敗
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表的外鍵約束，user_order表受userprofile表的約束

在user_order里面插入一條數據u_id為5用戶，在userprofile表里面根本沒有，所以插入數據失敗

先向userprofile表中插入記錄，再向user_order表中插入記錄就可以了

MariaDB [market]> insert into userprofile values(5,"XJ",1);			# 先向userprofile數據表中插入id為5的記錄，插入數據成功
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數據表中插入數據,成功
Query OK, 1 row affected (0.00 sec)

MariaDB [market]> select * from userprofile;				# 查詢userprofile數據表中的所有記錄
+----+------+-----+
| 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;					# 查詢user_order數據表中的所有記錄
+------+------+----------+-------+
| 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：通過alter table創建外鍵和級聯更新，級聯刪除

語法：

alter table 數據表名稱 add 
	[constraint [約束名稱] ]  foreign key (外鍵字段,..) references 數據表(參照字段,...) 
	[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));			# 創建user_order1數據表，創建表時不使用外鍵約束
Query OK, 0 rows affected (0.11 sec)

MariaDB [market]> show create table user_order1;			# 查看user_order1數據表的創建信息，沒有外鍵約束
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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數據表，為user_order1數據表添加外鍵約束
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數據表的創建信息，已經添加了外鍵約束
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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.刪除外鍵

語法

alter table 數據表名稱 drop foreign key 約束（外鍵）名稱

例子：

MariaDB [market]> show create table user_order1;			# 查看user_order1數據表的創建信息，包含外鍵約束
+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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;			# 為user_order1數據表刪除外鍵約束，外鍵名稱必須與從`show create table user_order1`語句中查到的相同
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數據表的創建信息，外鍵約束已經被刪除了
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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.使用外鍵約束的條件

要想外鍵創建成功，必須滿足以下4個條件：

1、確保參照的表和字段存在。 
2、組成外鍵的字段被索引。 
3、必須使用type指定存儲引擎為：innodb.
4、外鍵字段和關聯字段，數據類型必須一致。

5.使用外鍵約束需要的注意事項

1.on delete cascade  on update cascade 添加級聯刪除和更新：
2.確保參照的表userprofile中id字段存在。
3.確保組成外鍵的字段u_id被索引
4.必須使用type指定存儲引擎為：innodb。
5.外鍵字段和關聯字段，數據類型必須一致。