外鍵

外鍵的用途是確保數(shù)據(jù)的完整性。它通常包括以下幾種：

1 實(shí)體完整性，確保每個(gè)實(shí)體是唯一的（通過主鍵來實(shí)施）

2 域完整性，確保屬性值只從一套特定可選的集合里選擇

3 關(guān)聯(lián)完整性，確保每個(gè)外鍵或是NULL（如果允許的話）或含有與相關(guān)主鍵值相配的值

1.什么是外鍵約束

與主鍵約束不同，創(chuàng)建外鍵約束不會(huì)自動(dòng)創(chuàng)建對(duì)應(yīng)的索引。 但是由于以下原因，對(duì)外鍵手動(dòng)創(chuàng)建索引通常是有用的：

• 當(dāng)在查詢中組合相關(guān)表中的數(shù)據(jù)時(shí)，經(jīng)常在聯(lián)接條件中使用外鍵列，方法是將一個(gè)表的外鍵約束中的一列或多列與另一個(gè)表中的主鍵列或唯一鍵列匹配。 索引使 數(shù)據(jù)庫(kù)引擎 可以在外鍵表中快速查找相關(guān)數(shù)據(jù)。 但是，創(chuàng)建此索引并不是必需的。 即使沒有對(duì)兩個(gè)相關(guān)表定義主鍵或外鍵約束，也可以對(duì)來自這兩個(gè)表中的數(shù)據(jù)進(jìn)行組合，但兩個(gè)表間的外鍵關(guān)系說明已用其鍵作為條件對(duì)其進(jìn)行了優(yōu)化，以便組合到查詢中。
• 對(duì)主鍵約束的更改可由相關(guān)表中的外鍵約束檢查。

外鍵約束(foreign key)就是表與表之間的某種約定的關(guān)系，由于這種關(guān)系的存在，我們能夠讓表與表之間的數(shù)據(jù)，更加的完整，關(guān)連性更強(qiáng)。

關(guān)于數(shù)據(jù)表的完整性和關(guān)連性，可以舉個(gè)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(gè)用戶有關(guān)的數(shù)據(jù)，就成了無(wú)頭數(shù)據(jù)了，不完整了。
2.如果我在訂單表里面，隨便插入了一條數(shù)據(jù)，這個(gè)訂單在用戶表里面，沒有與之對(duì)應(yīng)的用戶。這樣數(shù)據(jù)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(shù)據(jù)，或者刪除用戶的話，通過外鍵同樣刪除訂單表里面的數(shù)據(jù)，這樣也能讓數(shù)據(jù)完整。

通過外鍵約束，每次插入或更新數(shù)據(jù)表時(shí)，都會(huì)檢查數(shù)據(jù)的完整性。

2.創(chuàng)建外鍵約束

2.1 方法一：通過create table創(chuàng)建外鍵

語(yǔ)法：

create table 數(shù)據(jù)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(shù)的解釋：

RESTRICT: 拒絕對(duì)父表的刪除或更新操作。
CASCADE: 從父表刪除或更新且自動(dòng)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(jí)聯(lián)更新的意思，on delete cascade是級(jí)聯(lián)刪除的意思，意思就是說當(dāng)你更新或刪除主鍵表，那外鍵表也會(huì)跟隨一起更新或刪除。

精簡(jiǎn)化后的語(yǔ)法：

foreign key 當(dāng)前表的字段 references 外部表名 (關(guān)聯(lián)的字段） type=innodb 

2.1.1 插入測(cè)試數(shù)據(jù)

例子：我們創(chuàng)建一個(gè)數(shù)據(jù)庫(kù)，包含用戶信息表和訂單表

MariaDB [book]> create database market;  # 創(chuàng)建market數(shù)據(jù)庫(kù)
Query OK, 1 row affected (0.00 sec)

MariaDB [book]> use market;    # 使用market數(shù)據(jù)庫(kù)
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(chuàng)建userprofile數(shù)據(jù)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(chuàng)建user_order數(shù)據(jù)表，同時(shí)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(shù)據(jù)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(shù)據(jù)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(shù)據(jù)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(shù)據(jù)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(lián)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(cè)試級(jí)聯(lián)刪除

MariaDB [market]> delete from userprofile where id=1; # 刪除user表中id為1的數(shù)據(jù)
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(shù)據(jù)
+------+------+----------+-------+
| 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(cè)試級(jí)聯(lián)更新

更新數(shù)據(jù)之前的狀態(tài)

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

更新數(shù)據(jù)

MariaDB [market]> update userprofile set id=6 where id=2; # 把userprofile數(shù)據(jù)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(shù)據(jù)后的狀態(tài)

MariaDB [market]> select id,name,sex,money,o_id from userprofile,user_order where id=u_id; # 聯(lián)表查詢，可以看出表中已經(jīng)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(shù)據(jù)，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(shù)據(jù),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(cè)試數(shù)據(jù)完整性

MariaDB [market]> insert into user_order(u_id,username,money)values(5,"XJ",345); # 單獨(dú)向user_order數(shù)據(jù)表中插入數(shù)據(jù)，插入數(shù)據(jù)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(shù)據(jù)u_id為5用戶，在userprofile表里面根本沒有，所以插入數(shù)據(jù)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(shù)據(jù)表中插入id為5的記錄，插入數(shù)據(jù)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(shù)據(jù)表中插入數(shù)據(jù),成功
Query OK, 1 row affected (0.00 sec)

MariaDB [market]> select * from userprofile;  # 查詢userprofile數(shù)據(jù)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(shù)據(jù)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(chuàng)建外鍵和級(jí)聯(lián)更新，級(jí)聯(lián)刪除

語(yǔ)法：

alter table 數(shù)據(jù)表名稱 add 
 [constraint [約束名稱] ] foreign key (外鍵字段,..) references 數(shù)據(jù)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(chuàng)建user_order1數(shù)據(jù)表，創(chuàng)建表時(shí)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(shù)據(jù)表的創(chuàng)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(shù)據(jù)表，為user_order1數(shù)據(jù)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(shù)據(jù)表的創(chuàng)建信息，已經(jīng)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(yǔ)法

alter table 數(shù)據(jù)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(shù)據(jù)表的創(chuàng)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(shù)據(jù)表刪除外鍵約束，外鍵名稱必須與從`show create table user_order1`語(yǔ)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(shù)據(jù)表的創(chuàng)建信息，外鍵約束已經(jīng)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(chuàng)建成功，必須滿足以下4個(gè)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(chǔ)引擎為：innodb.
4、外鍵字段和關(guān)聯(lián)字段，數(shù)據(jù)類型必須一致。

5.使用外鍵約束需要的注意事項(xiàng)

1.on delete cascade  on update cascade 添加級(jí)聯(lián)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(chǔ)引擎為：innodb。
5.外鍵字段和關(guān)聯(lián)字段，數(shù)據(jù)類型必須一致。

總結(jié)

以上就是這篇文章的全部?jī)?nèi)容了，希望本文的內(nèi)容對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值，如果有疑問大家可以留言交流，謝謝大家對(duì)的支持。