数据库开发——MySQL——primary key

四，primary key

关系型数据库中的一条记录中有若干个属性，若其中某一个属性组（注意是组）能唯一标识一条记录，该属性组就可以成为一个主键。

例如

学生表（学号，姓名，性别，班级）    其中每个学生的学号是唯一的，学号就是一个主键，这叫单列主键    

课程表（课程编号，课程名，学分）    其中课程编号是唯一的，课程编号就是一个主键，这叫单列主键成绩    

表（学号，课程号，    成绩）成绩表中唯一一个属性无法唯一标识一条记录，学号和课程号的组合才可以唯一标识一条记录，所以学号和课程号的属性组是一个主键，这叫多列主键

成绩表中的学号不是成绩表的主键，但它和学生表中的学号相对应，并且学生表
中的学号是学生表的主键，则称成绩表中的学号是学生表的外键。

同理成绩表中的课程号是课程表的外键 primary key关键字用于定义列为主键。

单列主键

在某一个字段后用primary key

 create table t6(
     id int primary key,
     name varchar(20),
     number char(10));

 desc t6;

执行结果为：

mysql>  create table t6(     id int primary key,     name varchar(20),     number char(10));
Query OK, 0 rows affected (0.53 sec)

mysql> desc t6;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int(11)     | NO   | PRI | NULL    |       |
| name   | varchar(20) | YES  |     | NULL    |       |
| number | char(10)    | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

在所有前端后单独定义主键

create table t7 （
	id int，
	name varchar （20)，
	number char （10），
	constraint pk_id primary key（id)); ＃创建主键并为此命名

desc t7; 

执行结果为：

mysql> create table t7(
    -> id int,
    -> name varchar(20),
    -> number varchar(20),
    -> constraint pk_id primary key(id));
Query OK, 0 rows affected (0.68 sec)

mysql> desc t7;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int(11)     | NO   | PRI | NULL    |       |
| name   | varchar(20) | YES  |     | NULL    |       |
| number | varchar(20) | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

多列主键

mysql> create table t8(
     -> student_number char(10),
     -> class_number char(10),
     -> achievement int not null default 0,
     -> primary key(student_number, class_number));
 Query OK, 0 rows affected (0.73 sec)

 mysql> desc t8;
 +----------------+----------+------+-----+---------+-------+
 | Field          | Type     | Null | Key | Default | Extra |
 +----------------+----------+------+-----+---------+-------+
 | student_number | char(10) | NO   | PRI | NULL    |       |
 | class_number   | char(10) | NO   | PRI | NULL    |       |
 | achievement    | int(11)  | NO   |     | 0       |       |
 +----------------+----------+------+-----+---------+-------+
 3 rows in set (0.00 sec)

 mysql> insert into t8 values
     -> ("18023300", "186000", 100),
     -> ("19205200", "192000", 90);
 Query OK, 2 rows affected (0.18 sec)
 Records: 2  Duplicates: 0  Warnings: 0

 mysql> insert into t8 values("18023300", "186000", 90);
 ERROR 1062 (23000): Duplicate entry '18023300-186000' for key 'PRIMARY'

 mysql> select * from t8;
 +----------------+--------------+-------------+
 | student_number | class_number | achievement |
 +----------------+--------------+-------------+
 | 18023300       | 186000       |         100 |
 | 19205200       | 192000       |          90 |
 +----------------+--------------+-------------+
 2 rows in set (0.00 sec)

auto_increment

我们每次往表中插入数据的时候都得记住id，这样非常考验脑力，有的时候我们可能记得不是很清楚，那就容易报错，使用auto_increment约束字段会自动增长，被约束的字段必须同时被key约束。

 create table t9(
     id int primary key auto_increment,
     name varchar(20),
     sex enum("male", "female"));

 desc t9;

 insert into t9(name) values("Alex"),("Coco");
 select * from t9;

 insert into t9 values(5, "BeiBei", "female");
 select * from t9;

 delete from t9;
 select * from t9;

 insert into t9(name, sex) values("Alex", "male"),("Coco", "female");
 select * from t9;

 truncate t9;
 insert into t9(name, sex) values("Alex", "male"),("Coco", "female");
 select * from t9;

执行结果为：

mysql> create table t9(
    -> id int primary key auto_increment,    # 定义自增长字段
    -> name varchar(20),
    -> sex enum("male", "female"));
Query OK, 0 rows affected (1.60 sec)

mysql> desc t9;
+-------+-----------------------+------+-----+---------+----------------+
| Field | Type                  | Null | Key | Default | Extra          |
+-------+-----------------------+------+-----+---------+----------------+
| id    | int(11)               | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20)           | YES  |     | NULL    |                |
| sex   | enum('male','female') | YES  |     | NULL    |                |
+-------+-----------------------+------+-----+---------+----------------+
3 rows in set (0.05 sec)

mysql> insert into t9(name) values("Alex"),("Coco");
Query OK, 2 rows affected (0.31 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from t9;    # 不指定id，则自动增长
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | Alex | NULL |
|  2 | Coco | NULL |
+----+------+------+
2 rows in set (0.00 sec)

mysql> insert into t9 values(5, "BeiBei", "female");    # 也可指指定id
Query OK, 1 row affected (0.23 sec)

mysql> select * from t9;
+----+--------+--------+
| id | name   | sex    |
+----+--------+--------+
|  1 | Alex   | NULL   |
|  2 | Coco   | NULL   |
|  5 | BeiBei | female |
+----+--------+--------+
3 rows in set (0.00 sec)

mysql>  delete from t9;
Query OK, 3 rows affected (0.52 sec)

mysql> select * from t9;
Empty set (0.00 sec)

mysql> insert into t9(name, sex) values("Alex", "male"),("Coco", "female");
Query OK, 2 rows affected (0.18 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from t9;    # 对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长
+----+------+--------+
| id | name | sex    |
+----+------+--------+
|  6 | Alex | male   |
|  7 | Coco | female |
+----+------+--------+
2 rows in set (0.00 sec)

mysql> truncate t9;        # 应该用truncate清空表，比起delete一条一条地删除记录，truncate是直接清空表，在删除大表时用它
Query OK, 0 rows affected (0.87 sec)

mysql> insert into t9(name, sex) values("Alex", "male"),("Coco", "female");
Query OK, 2 rows affected (0.09 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from t9;
+----+------+--------+
| id | name | sex    |
+----+------+--------+
|  1 | Alex | male   |
|  2 | Coco | female |
+----+------+--------+
2 rows in set (0.00 sec)

创建完表后可以修改自增细分的初始值

create table t10(
	id int primary key auto_increment,
	name varchar(20),
	sex enum('male','female') default 'male');

alter table t10 auto_increment = 5;		# 修改步长

show create table t10;

insert into t10(name) values("Alex");

select * from t10;

show create table t10;

执行结果为：

mysql> create table t10(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> sex enum('male','female') default 'male');
Query OK, 0 rows affected (0.72 sec)

mysql> alter table t10 auto_increment = 5;
Query OK, 0 rows affected (0.33 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table t10;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                             |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t10   | CREATE TABLE `t10` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `sex` enum('male','female') DEFAULT 'male',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> insert into t10(name) values("Alex");
Query OK, 1 row affected (0.24 sec)

mysql> select * from t10;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  5 | Alex | male |
+----+------+------+
1 row in set (0.00 sec)

mysql> show create table t10;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                             |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t10   | CREATE TABLE `t10` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `sex` enum('male','female') DEFAULT 'male',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

查看MySQL的步长：

show session variables like "auto_inc%";

执行结果为：

mysql> show session variables like "auto_inc%";
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 1     |	# auto_increment_increment 表示步长
| auto_increment_offset    | 1     |	# auto_increment_offset    表示其实偏移量
+--------------------------+-------+
2 rows in set, 1 warning (0.15 sec)

可以通过下列命令设置步长：

set session auth_increment_increment=2 #修改会话级别的步长

set global auth_increment_increment=2 #修改全局级别的步长（所有会话都生效）

设置全局的起始偏移量和步长需要重新启动MySQL。

如果auto_increment_offset的值大于auto_increment_increment的值，则auto_increment_offset的值会被忽略。