mysql的外键约束
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创建表格:
mysql> create table province (
-> id smallint auto_increment key,
-> name varchar(10) NOT NULL
-> );
创建一个子表，命名为student，其中其pid引用来自province的id，
mysql> create table student (
-> id smallint primary key,
-> name varchar(10) NOT NULL,
-> pid smallint,
-> foreign key(pid) references province(id)
-> );
注意，此时pid的类型一定要与id的类型一致，若为数字，则要完全一致；若为字符，字符个数可以不一致。此时，id称为参照键。
查看自动索引:
接下来，我们查看一下两个表是否已经创建成功及其成员，如
mysql> show columns in student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | smallint(6) | NO | PRI | NULL | |
| name | varchar(10) | NO | | NULL | |
| pid | smallint(6) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> show columns in province;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | smallint(6) | NO | PRI | NULL | auto_increment |
| name | varchar(10) | NO | | NULL | |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
再看一下两个表的自动索引情况，
mysql> show indexes from province\G;
*************************** 1. row ***************************
Table: province
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)
mysql> show indexes from student\G;
*************************** 1. row ***************************
Table: student
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: student
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)
因而，我们看到student表中pid也已经创建了自动索引了。
外键约束的参照操作
外键约束主要有以下几大类：
cascade：从父表删除或更新且自动删除或更新字表中的匹配的内容；
set null：从父表删除或更新且子表中相匹配的内容设置为NULL，使用此功能必须保证字表列没有指定为NOT NULL；
restrict：拒绝对父表的删除或更新操作；
no action：标准SQL关键字，在mysql中与restrict相同；
下面再创建一个新表来说明情况，先删除之前的student表，如
mysql> drop table student;
Query OK, 0 rows affected (0.13 sec)
mysql> show tables;
+-----------------------+
| Tables_in_zzw_test_db |
+-----------------------+
| province |
| tb1 |
+-----------------------+
2 rows in set (0.00 sec)
接下来，重新创建一个student表，并且指定外键约束为cascade，
mysql> create table student (
-> id smallint primary key,
-> name varchar(10) NOT NULL,
-> pid smallint,
-> foreign key(pid) references province(id) on delete cascade
-> );
Query OK, 0 rows affected (0.33 sec)
接着，向province中添加数据，
mysql> insert province(name) values("A");
Query OK, 1 row affected (0.05 sec)
mysql> insert province(name) values("B");
Query OK, 1 row affected (0.06 sec)
mysql> insert province(name) values("C");
Query OK, 1 row affected (0.04 sec)
mysql> insert province(name) values("D");
Query OK, 1 row affected (0.03 sec)
mysql> select * from province;
+----+------+
| id | name |
+----+------+
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
+----+------+
4 rows in set (0.00 sec)
接着，向student中添加数据，如
mysql> insert student values(1234,"Tom",1);
Query OK, 1 row affected (0.06 sec)
mysql> insert student values(1235,"John",2);
Query OK, 1 row affected (0.03 sec)
mysql> insert student values(1236,"Mary",3);
Query OK, 1 row affected (0.04 sec)
mysql> select * from student;
+------+------+------+
| id | name | pid |
+------+------+------+
| 1234 | Tom | 1 |
| 1235 | John | 2 |
| 1236 | Mary | 3 |
+------+------+------+
3 rows in set (0.00 sec)
现在，要把province中id为3的一行删除掉，
mysql> delete from province where id=3;
Query OK, 1 row affected (0.04 sec)
mysql> select * from province;
+----+------+
| id | name |
+----+------+
| 1 | A |
| 2 | B |
| 4 | D |
+----+------+
3 rows in set (0.00 sec)
现在来查看删除过后，子表student中的数据是否受到了影响，
mysql> select * from student;
+------+------+------+
| id | name | pid |
+------+------+------+
| 1234 | Tom | 1 |
| 1235 | John | 2 |
+------+------+------+
2 rows in set (0.01 sec)
我们，发现pid=3的一行已经不存在了，即它也自动删除了，这就是我们on delete cascade的作用。
（注意，外键约束需要默认引擎为innodb引擎，linux环境下可以通过修改配置文件/usr/share/mysql/my-default.cnf来修改默认引擎）
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