數據庫開發——MySQL——primary key

四，primary key

關係型數據庫中的一條記錄中有若干個屬性，若其中某一個屬性組（注意是組）能唯一標識一條記錄，該屬性組就可以成爲一個主鍵。

例如

學生表（學號，姓名，性別，班級）    其中每個學生的學號是唯一的，學號就是一個主鍵，這叫單列主鍵    

課程表（課程編號，課程名，學分）    其中課程編號是唯一的，課程編號就是一個主鍵，這叫單列主鍵成績    

表（學號，課程號，    成績）成績表中唯一一個屬性無法唯一標識一條記錄，學號和課程號的組合纔可以唯一標識一條記錄，所以學號和課程號的屬性組是一個主鍵，這叫多列主鍵

成績表中的學號不是成績表的主鍵，但它和學生表中的學號相對應，並且學生表
中的學號是學生表的主鍵，則稱成績表中的學號是學生表的外鍵。

同理成績表中的課程號是課程表的外鍵 primary key關鍵字用於定義列爲主鍵。

單列主鍵

在某一個字段後用primary key

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

 desc t6;

執行結果爲：

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)

在所有前端後單獨定義主鍵

create table t7 （
	id int，
	name varchar （20)，
	number char （10），
	constraint pk_id primary key（id)); ＃創建主鍵併爲此命名

desc t7; 

執行結果爲：

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)

多列主鍵

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

我們每次往表中插入數據的時候都得記住id，這樣非常考驗腦力，有的時候我們可能記得不是很清楚，那就容易報錯，使用auto_increment約束字段會自動增長，被約束的字段必須同時被key約束。

 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;

執行結果爲：

mysql> create table t9(
    -> id int primary key auto_increment,    # 定義自增長字段
    -> 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，則自動增長
+----+------+------+
| 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;    # 對於自增的字段，在用delete刪除後，再插入值，該字段仍按照刪除前的位置繼續增長
+----+------+--------+
| id | name | sex    |
+----+------+--------+
|  6 | Alex | male   |
|  7 | Coco | female |
+----+------+--------+
2 rows in set (0.00 sec)

mysql> truncate t9;        # 應該用truncate清空表，比起delete一條一條地刪除記錄，truncate是直接清空表，在刪除大表時用它
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)

創建完表後可以修改自增細分的初始值

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

alter table t10 auto_increment = 5;		# 修改步長

show create table t10;

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

select * from t10;

show create table t10;

執行結果爲：

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的步長：

show session variables like "auto_inc%";

執行結果爲：

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

可以通過下列命令設置步長：

set session auth_increment_increment=2 #修改會話級別的步長

set global auth_increment_increment=2 #修改全局級別的步長（所有會話都生效）

設置全局的起始偏移量和步長需要重新啓動MySQL。

如果auto_increment_offset的值大於auto_increment_increment的值，則auto_increment_offset的值會被忽略。

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