1、MySQL外键约束的要求解析
约束保证数据的完整性和一致性
约束分为表级约束和列级约束:对一个数据列建立的约束,称为列级约束;对多个数据列建立的约束,称为表级约束。列级约束既可以在列定义时声明,也可以在列定义后声明。表级约束只能列定义后声明。
约束类型包括(按功能分类):NOT NULL,PRIMARY KEY,UNIQUE KEY,DEFAULT,FOREIGN KEY
外键约束(实现一对一或一对多关系)要求:
父表与子表必须具有相同的存储引擎,而且禁用使用临时表
数据表的存储引擎必须为InnoDB
外键列与参照列必须具有相似的数据类型,其中数字的长度或有无符号位必须相同,而字符的长度可以不同。
mysql> use test;Database changed
父表:
mysql> CREATE TABLE provinces(-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> pname VARCHAR(20) NULL);
Query OK, 0 rows affected (0.02 sec)
mysql> SHOW CREATE TABLE provinces;
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| provinces | CREATE TABLE `provinces` (
`id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`pname` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
错误的子表演示:(参照列)
mysql> CREATE TABLE users(
-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> username VARCHAR(10) NOT NULL,
-> pid BIGINT,
-> FOREIGN KEY(pid) REFERENCES provinces(id)
-> );
ERROR 1005 (HY000): Can't create table 'test.users' (errno: 150)
子表:(外键列)
mysql> CREATE TABLE users(
-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> username VARCHAR(10) NOT NULL,
-> pid SMALLINT UNSIGNED,
-> FOREIGN KEY(pid) REFERENCES provinces(id)
-> );
Query OK, 0 rows affected (0.02 sec)
显示索引:
mysql> SHOW INDEXES FROM provinces;
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| provinces | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
参照列以网格的形式展示:
mysql> SHOW INDEXES FROM provinces\G;
*************************** 1. row ***************************
Table: provinces
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
1 row in set (0.00 sec)
ERROR:
No query specified
外键列索引:(外键列不存在索引的话,会自动创建)
mysql> SHOW INDEXES FROM users\G;
*************************** 1. row ***************************
Table: users
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 2. row ***************************
Table: users
Non_unique: 1
Key_name: pid
Seq_in_index: 1
Column_name: pid
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
2 rows in set (0.00 sec)
ERROR:
No query specified
显示users的数据表:
2、外键约束的参照操作
(1)CASCADE 从父表删除或更新且自动删除或更新子表中的匹配
mysql> CREATE TABLE users1( -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY, -> username VARCHAR(10) NOT NULL, -> pid SMALLINT UNSIGNED, -> FOREIGN KEY(pid) REFERENCES provinces(id) ON DELETE CASCADE -> ); Query OK, 0 rows affected (0.01 sec) mysql> INSERT provinces(pname) VALUES('A'); Query OK, 1 row affected (0.00 sec) mysql> INSERT provinces(pname) VALUES('B'); Query OK, 1 row affected (0.00 sec) mysql> INSERT provinces(pname) VALUES('C'); Query OK, 1 row affected (0.00 sec) mysql> SELECT * FROM provinces; +----+-------+ | id | pname | +----+-------+ | 1 | A | | 2 | B | | 3 | C | +----+-------+ 3 rows in set (0.00 sec) mysql> INSERT users1(username,pid) VALUES('Tom',3); Query OK, 1 row affected (0.02 sec) mysql> INSERT users1(username,pid) VALUES('John',2); Query OK, 1 row affected (0.00 sec) mysql> SELECT * FROM users1; +----+----------+------+ | id | username | pid | +----+----------+------+ | 1 | Tom | 3 | | 2 | John | 2 | +----+----------+------+ 2 rows in set (0.00 sec)
删除操作的关联:
mysql> DELETE FROM provinces WHERE id=3; Query OK, 1 row affected (0.01 sec) mysql> SELECT * FROM provinces; +----+-------+ | id | pname | +----+-------+ | 1 | A | | 2 | B | +----+-------+ 2 rows in set (0.00 sec) mysql> SELECT * FROM users1; +----+----------+------+ | id | username | pid | +----+----------+------+ | 2 | John | 2 | +----+----------+------+ 1 row in set (0.00 sec)
(2)SET NULL 从父表删除或更新行,并设置子表中的外键列为NULL 。如果使用该选项,必须保证子表列没有指定NOT NULL (3)RESTRICT:拒绝对父表的删除或更新操作
(4)NO ACTION: 标准SQL的关键字,在MySQL中与RESTRICT相同。
总结:
在外键约束当中,在实际的开发过程中,我们很少使用物理的外键约束,很多都去使用逻辑的外键约束,因为物理的外键约束只有INNODB这种引擎才会支持,像我们另外的一种引擎MYISAM的引擎则不支持,反过来说,如果我想创建的数据表,假设存储引擎为MYISAM,而且又想使用外键约束的话,其实是不可能实现的,所以说,我们在实际的项目开发中,我们不去定义物理的外键,所谓的逻辑外键指的是就是我们在定义两张表的结构的时候,我们是按照存在的某种结构的方式去定义,但是不去使用FOREIGN KEY这个关键词来定义。
补充:
1.表级约束与列级约束 (1)对一个数据列建立的约束,称为列级约束 (2)对多个数据列建立的约束,称为表级约束 (3)列级约束既可以在列定义时声明,也可以在列定以后声明 (4)表级约束只能在列定义后声明 2.主键、外键、非空、唯一、default都是约束条件。主键、外键、唯一既可以作为表级约束,也可作为列级约束 ( not null, default )只有列级约束