MySQL系列(十一)--外鍵約束foreign key的基本使用

有些時候，為了保證數據的完整性，我們會選擇的使用外鍵約束，例如教師對應的表和課程表中老師的id，這種時候就要使用外鍵約束了。

PS：這里不考慮表結構設計，三范式與反范式等設計問題，基於MySQL8.0

語法：

constraint 外鍵名 foreign key 外鍵字段 references 主表名(關聯字段) [主表記錄刪除時的動作] [主表記錄更新時的動作]

constraint可以省略，只是用來指定外鍵名

例如：

CREATE TABLE test (
    course_id INT (11) NOT NULL AUTO_INCREMENT,
    NAME VARCHAR (30) DEFAULT NULL,
    PRIMARY KEY (course_id),
    CONSTRAINT cour_id_fk FOREIGN KEY (course_id) REFERENCES teacher (teacher_id)
);

或者通過alter添加：

alter table course add constraint course_id_fk foreign key (course_id) references teacher(teacher_id) on delete cascade on update cascade;

PS：關聯主表的column必須是索引，如果不是索引無法添加外鍵約束

做個測試：

mysql> CREATE TABLE test2 (
    -> course_id INT (11) NOT NULL AUTO_INCREMENT,
    -> identified_no INT(18) UNIQUE,
    -> NAME VARCHAR (30) DEFAULT NULL,
    -> PRIMARY KEY (course_id)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> CREATE TABLE test1 (
    -> course_id INT (11) NOT NULL AUTO_INCREMENT,
    -> identified_no INT(18) UNIQUE,
    -> NAME VARCHAR (30) DEFAULT NULL,
    -> PRIMARY KEY (course_id),
    -> CONSTRAINT cour_id1_fk FOREIGN KEY (NAME) REFERENCES test2 (NAME)
    -> );
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'cour_id1_fk' in the referenced table 'test2'
mysql> drop table if exists test2;
Query OK, 0 rows affected (0.02 sec)

mysql> CREATE TABLE test2 (
    -> course_id INT (11) NOT NULL AUTO_INCREMENT,
    -> identified_no INT(18) UNIQUE,
    -> NAME VARCHAR (30) DEFAULT NULL,
    -> PRIMARY KEY (course_id),
    -> INDEX(NAME)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> CREATE TABLE test1 (
    -> course_id INT (11) NOT NULL AUTO_INCREMENT,
    -> identified_no INT(18) UNIQUE,
    -> NAME VARCHAR (30) DEFAULT NULL,
    -> PRIMARY KEY (course_id),
    -> CONSTRAINT cour_id1_fk FOREIGN KEY (NAME) REFERENCES test2 (NAME)
    -> );
Query OK, 0 rows affected (0.04 sec)

結果：name列如果不是索引，無法作為外鍵的引用列，當我們添加name為索引，發現添加外鍵約束成功

外鍵約束的方式有四種：

1、cascade：

　　級聯方式，刪除/更新父表的某條記錄，子表中引用該值的記錄會自動被刪除/更新

CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1002 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
CREATE TABLE `course` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  `teacher_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `course_id_fk` (`teacher_id`),
  CONSTRAINT `course_id_fk` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

mysql> select * from teacher;
+------+-------+
| id   | name  |
+------+-------+
| 1001 | sam   |
| 1002 | jesen |
+------+-------+
2 rows in set (0.00 sec)

mysql> select * from course;
+----+--------+------------+
| id | name   | teacher_id |
+----+--------+------------+
|  1 | 語文   |       1001 |
|  2 | 數學   |       1002 |
+----+--------+------------+
2 rows in set (0.00 sec)

mysql> update teacher set id=1 where id=1001;　　//更新主表的id，從表的外鍵值會更新
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from course;
+----+--------+------------+
| id | name   | teacher_id |
+----+--------+------------+
|  1 | 語文   |          1 |
|  2 | 數學   |       1002 |
+----+--------+------------+
2 rows in set (0.00 sec)

mysql> delete from teacher where id=1;　　//刪除主表的id，從表外鍵值對應的那條數據也會刪除
Query OK, 1 row affected (0.00 sec)

mysql> select * from course;
+----+--------+------------+
| id | name   | teacher_id |
+----+--------+------------+
|  2 | 數學   |       1002 |
+----+--------+------------+
1 row in set (0.00 sec)

mysql> update course set teacher_id=2 where id =1;　　//不能直接更新從表的外鍵
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`mysql`.`course`, CONSTRAINT `course_id_fk` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)

2、set null：

　　設置為null。主表主鍵值被更新或刪除，從表的外鍵被設置為null。但注意，要求該外鍵列，沒有not null屬性約束。

先刪除外鍵，后重建：

mysql> alter table course drop foreign key course_id_fk;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table course add constraint course_id_fk foreign key (teacher_id) references teacher(id) on delete set null on update set null;
Query OK, 1 row affected (0.06 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from course;
+----+--------+------------+
| id | name   | teacher_id |
+----+--------+------------+
|  1 | 數學   |       1002 |
+----+--------+------------+
1 row in set (0.00 sec)

mysql> update teacher set id=1001;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from course;
+----+--------+------------+
| id | name   | teacher_id |
+----+--------+------------+
|  1 | 數學   |       NULL |
+----+--------+------------+
1 row in set (0.00 sec)
mysql> delete from teacher ;
Query OK, 1 row affected (0.00 sec)

mysql> select * from course;
+----+--------+------------+
| id | name   | teacher_id |
+----+--------+------------+
|  1 | 數學   |       NULL |
+----+--------+------------+
1 row in set (0.00 sec)

no action/restrict：

　　禁止模式，拒絕父表刪除和更新

mysql> alter table course drop foreign key course_id_fk;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table course add constraint course_id_fk foreign key (teacher_id) references teacher(id) on delete no action on update restrict;
Query OK, 1 row affected (0.06 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> update teacher set id=10012;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`mysql`.`course`, CONSTRAINT `course_id_fk` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`) ON UPDATE RESTRICT)

mysql> delete from teacher;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`mysql`.`course`, CONSTRAINT `course_id_fk` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`) ON UPDATE RESTRICT)

默認：也是禁止模式

alter table course add constraint course_id_fk foreign key (teacher_id) references teacher(id);