mysql主键和外键

什么是主键？

作查询字段的适合做索引，加快查找速度
一个表中只能有一个primary key字段
对应的字段不容许重复，且不容许赋NULL值
如果有多个字段都为PRIMARY KEY，称为复合主键，必须一起创建。
主键字段的KEY标志是PRI
通常与AUTO——INCREMENT  ----字段值自增长 ++
经常把表中能够唯一标示记录的字段设置为主键字段

添加主键的两种方式
 
mysql> create table t17(
    -> stu_id char(9) primary key,
    -> name char(10)
    -> );
Query OK, 0 rows affected (0.25 sec)   ----方式一

mysql> create table t18(
    -> stu_id char(9),
    -> name char(10),
    -> primary key(stu_id)
    -> );
Query OK, 0 rows affected (0.21 sec)    ----方式二

mysql> desc t18;
+--------+----------+------+-----+---------+-------+
| Field  | Type     | Null | Key | Default | Extra |
+--------+----------+------+-----+---------+-------+
| stu_id | char(9)  | NO   | PRI | NULL    |       |
| name   | char(10) | YES  |     | NULL    |       |
+--------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> insert into t17 values("1","ss");
Query OK, 1 row affected (0.12 sec)

mysql> insert into t17 values("1","ss");    
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'   ----主键约束
mysql> insert into t17 values("1","s");
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

mysql> alter table t17 drop primary key;          -----删除主键
Query OK, 1 row affected (0.46 sec)
Records: 1  Duplicates: 0  Warnings: 0
mysql> alter table t17 add primary key(stu_id);   ----添加主键
Query OK, 0 rows affected (0.48 sec)
Records: 0  Duplicates: 0  Warnings: 0

添加复合主键
mysql> create table t19(
    -> cip char(15),
    -> serport smallint(2),
    -> status enum("yes","no"),
    -> primary key(cip,serport));
Query OK, 0 rows affected (0.43 sec)

mysql> insert into t19 values("1.1.1.1",22,"no");
Query OK, 1 row affected (0.17 sec)

mysql> insert into t19 values("1.1.1.1",21,"yes");
Query OK, 1 row affected (0.10 sec)

删除主键
mysql> alter table t19 drop primary key;
Query OK, 2 rows affected (0.62 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> desc t19;
+---------+------------------+------+-----+---------+-------+
| Field   | Type             | Null | Key | Default | Extra |
+---------+------------------+------+-----+---------+-------+
| cip     | char(15)         | NO   |     | NULL    |       |
| serport | smallint(2)      | NO   |     | NULL    |       |
| status  | enum('yes','no') | YES  |     | NULL    |       |
+---------+------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

添加主键
mysql> alter table t19 add primary key(cip,serport);
Query OK, 0 rows affected (0.51 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t19;
+---------+------------------+------+-----+---------+-------+
| Field   | Type             | Null | Key | Default | Extra |
+---------+------------------+------+-----+---------+-------+
| cip     | char(15)         | NO   | PRI | NULL    |       |
| serport | smallint(2)      | NO   | PRI | NULL    |       |
| status  | enum('yes','no') | YES  |     | NULL    |       |
+---------+------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

自增长的两个条件：必须是主键且最好是整数类型的

mysql> create table t20( 
    -> stu_id int(2) primary key
    -> auto_increment,
    -> name char(10),
    -> age tinyint(2)
    -> );
Query OK, 0 rows affected (1.73 sec)

mysql> desc t20;
+--------+------------+------+-----+---------+----------------+
| Field  | Type       | Null | Key | Default | Extra          |
+--------+------------+------+-----+---------+----------------+
| stu_id | int(2)     | NO   | PRI | NULL    | auto_increment |
| name   | char(10)   | YES  |     | NULL    |                |
| age    | tinyint(2) | YES  |     | NULL    |                |
+--------+------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> insert into t20(name,age)values("bob",19);
Query OK, 1 row affected (0.30 sec)

mysql> select * from t20;
+--------+------+------+
| stu_id | name | age  |
+--------+------+------+
|      1 | bob  |   19 |
+--------+------+------+
1 row in set (0.00 sec)

mysql> insert into t20 values(null,"bosb",29);
Query OK, 1 row affected (0.64 sec)

mysql> select * from t20;
+--------+------+------+
| stu_id | name | age  |
+--------+------+------+
|      1 | bob  |   19 |
|      2 | bosb |   29 |
+--------+------+------+
2 rows in set (0.00 sec)

当自定义起自增长的数值时，再次的给该表赋值的时候，则在最大的值进行自动增长
mysql> select * from t20;
+--------+------+------+
| stu_id | name | age  |
+--------+------+------+
|      1 | bob  |   19 |
|      2 | bosb |   29 |
|     19 | zsb  |   29 |
+--------+------+------+
3 rows in set (0.00 sec)

mysql> insert into t20 values(NULL,"assb",39);
Query OK, 1 row affected (0.09 sec)

mysql> select * from t20;
+--------+------+------+
| stu_id | name | age  |
+--------+------+------+
|      1 | bob  |   19 |
|      2 | bosb |   29 |
|     19 | zsb  |   29 |
|     20 | assb |   39 |
+--------+------+------+
4 rows in set (0.00 sec)

什么时外键？
--当前表字段的值在另一个表中字段值的范围内选择。
使用外键的条件
--表的存储引擎必须是innodb
--字段类型要一致
--被参考字段必须要是索引类型的一种（primary key）

mysql> create table gzb(
    -> ya_id int(2) primary key auto_increment,      ----创建可以参照的表
    -> name char(15),
    -> bumen char(20)
    -> )engine=innodb;
Query OK, 0 rows affected (0.29 sec)

mysql> insert into ygb(name,bumen)values("bob","tea"),("jack","tea");
Query OK, 2 rows affected (0.10 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from ygb;
+-------+------+-------+
| ya_id | name | bumen |
+-------+------+-------+
|     1 | bob  | tea   |
|     2 | jack | tea   |
+-------+------+-------+
2 rows in set (0.00 sec)
 

mysql> create table gzb(
    -> gz_id int(2),
    -> name char(15),
    -> pay float(7,2),
    -> bumen char(20),
    -> foreign key(gz_id) references ygb(ya_id)  ------设置外键
    -> on update cascade           -----同步的更新
    -> on delete cascade            -----同步的删除
    -> )
    -> engine=innodb;
mysql> show  create table gzb;     -----查看外键的信息
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                         |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| gzb   | CREATE TABLE `gzb` (
  `gz_id` int(2) DEFAULT NULL,
  `name` char(15) DEFAULT NULL,
  `pay` float(7,2) DEFAULT NULL,
  `bumen` char(20) DEFAULT NULL,
  KEY `gz_id` (`gz_id`),
  CONSTRAINT `gzb_ibfk_1` FOREIGN KEY (`gz_id`) REFERENCES `ygb` (`ya_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> insert into gzb values (1,"zhu",1333,"tea");    -----必须依赖参照物的表
Query OK, 1 row affected (0.06 sec)

mysql> insert into gzb values (5,"zhu",1333,"tea");     -----必须依赖参照物的表，否则会报错
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`zhuhaiyan`.`gzb`, CONSTRAINT `gzb_ibfk_1` FOREIGN KEY (`gz_id`) REFERENCES `ygb` (`ya_id`) ON DELETE CASCADE ON UPDATE CASCADE)