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myql之完整性约束

时间:2018-05-08 19:41:37      阅读:149      评论:0      收藏:0      [点我收藏+]

标签:选项   student   ddc   隐藏   重复   遇到   strong   records   isp   

一 介绍

约束条件与数据类型的宽度一样,都是可选参数

作用:用于保证数据的完整性和一致性
主要分为:

PRIMARY KEY (PK)    标识该字段为该表的主键,可以唯一的标识记录
FOREIGN KEY (FK)    标识该字段为该表的外键
NOT NULL    标识该字段不能为空
UNIQUE KEY (UK)    标识该字段的值是唯一的
AUTO_INCREMENT    标识该字段的值自动增长(整数类型,而且为主键)
DEFAULT    为该字段设置默认值

UNSIGNED 无符号
ZEROFILL 使用0填充

注意:

1. 是否允许为空,默认NULL,可设置NOT NULL,字段不允许为空,必须赋值
2. 字段是否有默认值,缺省的默认值是NULL,如果插入记录时不给字段赋值,此字段使用默认值
sex enum(male,female) not null default male
age int unsigned NOT NULL default 20 必须为正值(无符号) 不允许为空 默认是20
3. 是否是key
主键 primary key
外键 foreign key
索引 (index,unique...)

二 not null与default

是否可空,null表示空,非字符串
not null - 不可空
null - 可空


默认值,创建列时可以指定默认值,当插入数据时如果未主动设置,则自动添加默认值
create table tb1(
id int not null defalut 2,
num int not null
)

技术分享图片
==================not null====================
mysql> create table t1(id int); #id字段默认可以插入空
mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
mysql> insert into t1 values(); #可以插入空


mysql> create table t2(id int not null); #设置字段id不为空
mysql> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
mysql> insert into t2 values(); #不能插入空
ERROR 1364 (HY000): Field id doesnt have a default value



==================default====================
#设置id字段有默认值后,则无论id字段是null还是not null,都可以插入空,插入空默认填入default指定的默认值
mysql> create table t3(id int default 1);
mysql> alter table t3 modify id int not null default 1;



==================综合练习====================
mysql> create table student(
    -> name varchar(20) not null,
    -> age int(3) unsigned not null default 18,
    -> sex enum(male,female) default male,
    -> hobby set(play,study,read,music) default play,music
    -> );
mysql> desc student;
+-------+------------------------------------+------+-----+------------+-------+
| Field | Type                               | Null | Key | Default    | Extra |
+-------+------------------------------------+------+-----+------------+-------+
| name  | varchar(20)                        | NO   |     | NULL       |       |
| age   | int(3) unsigned                    | NO   |     | 18         |       |
| sex   | enum(male,female)              | YES  |     | male       |       |
| hobby | set(play,study,read,music) | YES  |     | play,music |       |
+-------+------------------------------------+------+-----+------------+-------+
mysql> insert into student(name) values(egon);
mysql> select * from student;
+------+-----+------+------------+
| name | age | sex  | hobby      |
+------+-----+------+------------+
| egon |  18 | male | play,music |
+------+-----+------+------------+
验证

三 unique (key)(*****)

技术分享图片
============设置唯一约束 UNIQUE===============
方法一:
create table department1(
id int,
name varchar(20) unique,
comment varchar(100)
);


方法二:
create table department2(
id int,
name varchar(20),
comment varchar(100),
constraint uk_name unique(name)
);


mysql> insert into department1 values(1,IT,技术);
Query OK, 1 row affected (0.00 sec)
mysql> insert into department1 values(1,IT,技术);
ERROR 1062 (23000): Duplicate entry IT for key name
唯一约束unique
技术分享图片
mysql> create table t1(id int not null unique);
Query OK, 0 rows affected (0.02 sec)

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
row in set (0.00 sec)

这是由于mysql内部存储和查找机制决定的,mysql内部存储查找都是基于索引的,这就要求每一个表都必须有一个主键primary key,mysql就会依据这个key来组织数据,这样可以优化查找速度,相当于目录,
也就是说,如果你创建表是没有指定主键,那么mysql会从上自下自动查找你的定义字段,只要遇到非空和唯一字段,就会自动将其设置为主键,而如果非空且唯一字段也没有的话,那么mysql会默认用自带的隐藏主键,但是这样的话就丧失优化查找的能力,大大降低了效率。
not null+unique的化学反应
技术分享图片
create table service(
id int primary key auto_increment,
name varchar(20),
host varchar(15) not null,
port int not null,
unique(host,port) #联合唯一
);

mysql> insert into service values
    -> (1,nginx,192.168.0.10,80),
    -> (2,haproxy,192.168.0.20,80),
    -> (3,mysql,192.168.0.30,3306)
    -> ;
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into service(name,host,port) values(nginx,192.168.0.10,80);
ERROR 1062 (23000): Duplicate entry 192.168.0.10-80 for key host

联合的任意一方都可以单独重复,但是双方组合起来的数据不能重复,这就是联合唯一!
联合唯一

四 primary key(******)

从约束角度看primary key字段的值不为空且唯一,那我们直接使用not null+unique不就可以了吗,要它干什么?

主键primary key是innodb存储引擎组织数据的依据,innodb称之为索引组织表,一张表中必须有且只有一个主键。

一个表中可以:

单列做主键
多列做主键(复合主键)

技术分享图片
============单列做主键===============
#方法一:not null+unique
create table department1(
id int not null unique, #主键
name varchar(20) not null unique,
comment varchar(100)
);

mysql> desc department1;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | NO   | UNI | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

#方法二:在某一个字段后用primary key
create table department2(
id int primary key, #主键
name varchar(20),
comment varchar(100)
);

mysql> desc department2;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.00 sec)

#方法三:在所有字段后单独定义primary key
create table department3(
id int,
name varchar(20),
comment varchar(100),
constraint pk_name primary key(id); #创建主键并为其命名pk_name

mysql> desc department3;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)
主键产生的三者方式
技术分享图片
==================多列做主键================
create table service(
ip varchar(15),
port char(5),
service_name varchar(10) not null,
primary key(ip,port)
);


mysql> desc service;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip           | varchar(15) | NO   | PRI | NULL    |       |
| port         | char(5)     | NO   | PRI | NULL    |       |
| service_name | varchar(10) | NO   |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

mysql> insert into service values
    -> (172.16.45.10,3306,mysqld),
    -> (172.16.45.11,3306,mariadb)
    -> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> insert into service values (172.16.45.10,3306,nginx);
ERROR 1062 (23000): Duplicate entry 172.16.45.10-3306 for key PRIMARY
联合主键

五 auto_increment

约束字段为自动增长,被约束的字段必须同时被key约束(unique key,primary key)

技术分享图片
#不指定id,则自动增长
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum(male,female) default male
);

mysql> desc student;
+-------+-----------------------+------+-----+---------+----------------+
| Field | Type                  | Null | Key | Default | Extra          |
+-------+-----------------------+------+-----+---------+----------------+
| id    | int(11)               | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20)           | YES  |     | NULL    |                |
| sex   | enum(male,female) | YES  |     | male    |                |
+-------+-----------------------+------+-----+---------+----------------+
mysql> insert into student(name) values
    -> (egon),
    -> (alex)
    -> ;

mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | egon | male |
|  2 | alex | male |
+----+------+------+


#也可以指定id
mysql> insert into student values(4,asb,female);
Query OK, 1 row affected (0.00 sec)

mysql> insert into student values(7,wsb,female);
Query OK, 1 row affected (0.00 sec)

mysql> select * from student;
+----+------+--------+
| id | name | sex    |
+----+------+--------+
|  1 | egon | male   |
|  2 | alex | male   |
|  4 | asb  | female |
|  7 | wsb  | female |
+----+------+--------+


#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
mysql> delete from student;
Query OK, 4 rows affected (0.00 sec)

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

mysql> insert into student(name) values(ysb);
mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  8 | ysb  | male |
+----+------+------+

#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
mysql> truncate student;
Query OK, 0 rows affected (0.01 sec)

mysql> insert into student(name) values(egon);
Query OK, 1 row affected (0.01 sec)

mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | egon | male |
+----+------+------+
row in set (0.00 sec)
自增在用delete删除后会发生什么?

了解知识

技术分享图片
#在创建完表后,修改自增字段的起始值
mysql> create table student(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> sex enum(male,female) default male
    -> );

mysql> alter table student auto_increment=3;

mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

mysql> insert into student(name) values(egon);
Query OK, 1 row affected (0.01 sec)

mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  3 | egon | male |
+----+------+------+
row in set (0.00 sec)

mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8


#也可以创建表时指定auto_increment的初始值,注意初始值的设置为表选项,应该放到括号外
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum(male,female) default male
)auto_increment=3;




#设置步长
sqlserver:自增步长
    基于表级别
    create table t1(
        id int。。。
    )engine=innodb,auto_increment=2 步长=2 default charset=utf8

mysql自增的步长:
    show session variables like auto_inc%;
    
    #基于会话级别
    set session auth_increment_increment=2 #修改会话级别的步长

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


#!!!注意了注意了注意了!!!
If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored. 
翻译:如果auto_increment_offset的值大于auto_increment_increment的值,则auto_increment_offset的值会被忽略 ,这相当于第一步步子就迈大了,扯着了蛋
比如:设置auto_increment_offset=3,auto_increment_increment=2




mysql> set global auto_increment_increment=5;
Query OK, 0 rows affected (0.00 sec)

mysql> set global auto_increment_offset=3;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like auto_incre%; #需要退出重新登录
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 1     |
| auto_increment_offset    | 1     |
+--------------------------+-------+



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

mysql> insert into student(name) values(egon1),(egon2),(egon3);
mysql> select * from student;
+----+-------+------+
| id | name  | sex  |
+----+-------+------+
|  3 | egon1 | male |
|  8 | egon2 | male |
| 13 | egon3 | male |
+----+-------+------+
步长:auto_increment_increment,起始偏移量:auto_increment_offset

六 总结

not null与default

技术分享图片

 unique唯一

 技术分享图片

主键

#primary key:单单从约束角度去看,primary key就等同于not null unique
#强调(******)
1、一张表中必须有,并且只能有一个主键
2、一张表中都应该有一个id字段,而且应该把id字段做成主键

create table t17(
id int primary key,
name char(16),
age int,
sex char(6)
)engine=innodb;

#联合主键
create table t19(
ip char(15),
port int,
primary key(ip,port)
);

# primary key auto_increment
create table t20(
id int primary key auto_increment,
name char(16)
)engine=innodb;


# auto_increment注意点:
1、通常与primary key连用,而且通常是给id字段加
2、auto_incremnt只能给被定义成key(unique key,primary key)的字段加

 

myql之完整性约束

标签:选项   student   ddc   隐藏   重复   遇到   strong   records   isp   

原文地址:https://www.cnblogs.com/Dominic-Ji/p/9009674.html

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