MySQL 完整性约束

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角按，可以唯一的表示记录POREIGN KEY (FK)  #标识该字段为该表的外键NOT NULL #表示该字段不能为空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 #该字段设置默认值UNSIGNED #无符号ZEROFILL  #使用0填充

说明：

 

#1.是否允许为空，默认NULL，可设置NOT NULL,字段不允许为空，必赋值#2.字段是否有默认值，缺省的默认值是NULL，如果想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'# 必须为正值（无符号）不允许为空 默认时20age int unsigned NOT NULL default 20#30是否时key主键 primary key外键 foreign  key索引  （index，unique。。。）

 二、not null 与default

是否可空，null标识空，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（nid int not null default 2，num int not null）；

 

验证：

mysql> create table t11(id int);# id字段默认可以为空Query OK, 0 rows affected (0.05 sec)mysql> desc t11;+-------+---------+------+-----+---------+-------+| Field | Type| Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| id| int(11) | YES|  | NULL||+-------+---------+------+-----+---------+-------+1 row in set (0.03 sec)mysql> insert into t11 values(); #给t11表插一个空的值Query OK, 1 row affected (0.00 sec)#查询结果如下mysql> select * from t11;+------+| id|+------+| NULL |+------+1 row in set (0.00 sec)

 

验证2：

mysql> create table t12(id int not null);#设置字段id不为空Query OK, 0 rows affected (0.03 sec)mysql> desc t12;+-------+---------+------+-----+---------+-------+| Field | Type| Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| id| int(11) | NO| | NULL||+-------+---------+------+-----+---------+-------+1 row in set (0.01 sec)mysql> insert into t12 values();#不能插入空ERROR 1364 (HY000): Field 'id' doesn't have a default value

 

验证3：

mysql> create table student2(-> id int not null,-> name varchar(50) not null,-> age int(3) unsigned not null default 18,-> sex enum('male','female') default 'male',-> fav set('smoke','drink','tangtou') default 'drink,tangtou'-> );Query OK, 0 rows affected (0.01 sec)# 只插入了not null约束条件的字段对应的值mysql> insert into student2(id,name) values(1,'mjj');Query OK, 1 row affected (0.00 sec)# 查询结果如下mysql> select * from student2;+----+------+-----+------+---------------+| id | name | age | sex  | fav|+----+------+-----+------+---------------+|  1 | mjj  |  18 | male | drink,tangtou |+----+------+-----+------+---------------+1 row in set (0.00 sec)

 

3.unique

中文翻译：不同的。在MySQL中成为单列唯一

举例说明：创建公司部门表（每个公司都有唯一的部门）。

mysql> create table department(-> id int,-> name char(10)-> );Query OK, 0 rows affected (0.01 sec)mysql> insert into department values(1,'IT'),(2,'IT');Query OK, 2 rows affected (0.00 sec)Records: 2  Duplicates: 0  Waings: 0mysql> select * from department;+------+------+| id| name |+------+------+|1 | IT||2 | IT|+------+------+2 rows in set (0.00 sec)# 发现： 同时插入两个IT部门也是可以的，但这是不合理的，所以我们要设置name字段为unique 解决这种不合理的现象。

 

接下来，使用约束条件unique，来对公司部门的字段进行设置。

#第一种创建unique的方式#例子1：create table department(id int,name char(10) unique);mysql> insert into department values(1,'it'),(2,'it');ERROR 1062 (23000): Duplicate entry 'it' for key 'name'#例子2：create table department(id int unique,name char(10) unique);insert into department values(1,'it'),(2,'sale');#第二种创建unique的方式create table department(id int,name char(10) ,unique(id),unique(name));insert into department values(1,'it'),(2,'sale');

 

联合唯一：

# 创建services表mysql> create table services(-> id int,-> ip char(15),-> port int,-> unique(id),-> unique(ip,port)-> );Query OK, 0 rows affected (0.05 sec)mysql> desc services;+-------+----------+------+-----+---------+-------+| Field | Type  | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| id| int(11)| YES| UNI  | NULL| || ip| char(15) | YES| MUL  | NULL| || port| int(11) | YES|  | NULL| |+-------+----------+------+-----+---------+-------+3 rows in set (0.01 sec)#联合唯一，只要两列记录，有一列不同，既符合联合唯一的约束mysql> insert into services values-> (1,'192,168,11,23',80),-> (2,'192,168,11,23',81),-> (3,'192,168,11,25',80);Query OK, 3 rows affected (0.01 sec)Records: 3  Duplicates: 0  Waings: 0mysql> select * from services;+------+---------------+------+| id| ip| port |+------+---------------+------+|1 | 192,168,11,23 |80 ||2 | 192,168,11,23 |81 ||3 | 192,168,11,25 |80 |+------+---------------+------+3 rows in set (0.00 sec)mysql> insert into services values (4,'192,168,11,23',80);ERROR 1062 (23000): Duplicate entry '192,168,11,23-80' for key 'ip'

 

4.primary key

一个表可以：

单列做主键

多列做主键（复合主键）

约束：等价于 not null unique，字段的值不能为空，且唯一

存储引擎默认时（innodb）：对于innodb存储引擎来说，一张表必须有一个主键

单列主键

# 创建t14表，为id字段设置主键，唯一的不同的记录create table t14(id int primary key,name char(16));insert into t14 values(1,'xiaoma'),(2,'xiaohong');mysql> insert into t14 values(2,'wxxx');ERROR 1062 (23000): Duplicate entry '6' for key 'PRIMARY'#not null + unique的化学反应，相当于给id设置primary keycreate table t15(id int not null unique,name char(16));mysql> create table t15(-> id int not null unique,-> name char(16)-> );Query OK, 0 rows affected (0.01 sec)mysql> desc t15;+-------+----------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| id| int(11)  | NO | PRI | NULL| || name| char(16) | YES  | | NULL| |+-------+----------+------+-----+---------+-------+2 rows in set (0.02 sec)

 

复合主键：

create table t16(ip char(15),port int,primary key(ip,port));insert into t16 values('1.1.1.2',80),('1.1.1.2',81);

 

5.auto_increment

约束：约束的字段为自动增长，约束的字段必须同时被key约束

 

（重点）验证：

# 创建studentcreate 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||+-------+-----------------------+------+-----+---------+----------------+3 rows in set (0.17 sec)#插入记录mysql>  insert into student(name) values ('老白'),('小白');Query OK, 2 rows affected (0.01 sec)Records: 2  Duplicates: 0  Waings: 0mysql> select * from student;+----+--------+------+| id | name| sex  |+----+--------+------+|  1 | 老白| male ||  2 | 小白| male |+----+--------+------+2 rows in set (0.00 sec)

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 (0.01 sec)mysql> select * from student;+----+--------+--------+| id | name| sex|+----+--------+--------+|  1 | 老白| male||  2 | 小白| male||  4 | asb| female ||  7 | wsb| female |+----+--------+--------+4 rows in set (0.00 sec)# 再次插入一条不指定id的记录，会在之前的最后一条记录继续增长mysql>  insert into student(name) values ('大白');Query OK, 1 row affected (0.00 sec)mysql> select * from student;+----+--------+--------+| id | name| sex|+----+--------+--------+|  1 | 老白| male||  2 | 小白| male||  4 | asb| female ||  7 | wsb| female ||  8 | 大白| male|+----+--------+--------+5 rows in set (0.00 sec)

mysql> delete from student;Query OK, 5 rows affected (0.00 sec)mysql> select * from student;Empty set (0.00 sec)mysql> select * from student;Empty set (0.00 sec)mysql> insert into student(name) values('ysb');Query OK, 1 row affected (0.01 sec)mysql> select * from student;+----+------+------+| id | name | sex  |+----+------+------+|  9 | ysb  | male |+----+------+------+1 row in set (0.00 sec)#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 (0.03 sec)mysql>  insert into student(name) values('xiaobai');Query OK, 1 row affected (0.00 sec)mysql> select * from student;+----+---------+------+| id | name| sex  |+----+---------+------+|  1 | xiaobai | male |+----+---------+------+1 row in set (0.00 sec)

了解：

查看可用的 开头auto_inc的词mysql> show variables like 'auto_inc%';+--------------------------+-------+| Variable_name| Value |+--------------------------+-------+| auto_increment_increment | 1 || auto_increment_offset| 1 |+--------------------------+-------+2 rows in set (0.02 sec)# 步长auto_increment_increment,默认为1# 起始的偏移量auto_increment_offset, 默认是1 # 设置步长 为会话设置，只在本次连接中有效 set session auto_increment_increment=5; #全局设置步长 都有效。 set global auto_increment_increment=5; # 设置起始偏移量 set global  auto_increment_offset=3;#强调：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 # 设置完起始偏移量和步长之后，再次执行show variables like'auto_inc%';发现跟之前一样，必须先exit,再登录才有效。mysql> show variables like'auto_inc%';+--------------------------+-------+| Variable_name| Value |+--------------------------+-------+| auto_increment_increment | 5 || auto_increment_offset| 3 |+--------------------------+-------+2 rows in set (0.00 sec)#因为之前有一条记录id=1mysql> select * from student;+----+---------+------+| id | name| sex  |+----+---------+------+|  1 | xiaobai | male |+----+---------+------+1 row in set (0.00 sec)# 下次插入的时候，从起始位置3开始，每次插入记录id+5mysql> insert into student(name) values('ma1'),('ma2'),('ma3');Query OK, 3 rows affected (0.00 sec)Records: 3  Duplicates: 0  Waings: 0mysql> select * from student;+----+---------+------+| id | name| sex  |+----+---------+------+|  1 | xiaobai | male ||  3 | ma1 | male ||  8 | ma2 | male || 13 | ma3 | male |+----+---------+------+

清空表区分delete和truncate的区别：

delete from t1；#如果有自增id，新增的数据，仍然是以删除前的最后一样作为起始。

truncate table t1；数据量大，删除速度比上一条快，且直接从零开始。

6.foreign key

一 快速理解foreign key

之前在创建表的时候都是在一张表中添加记录，比如如下表：

 

公司有三个部门，但是有一个亿的员工，那意味着这个字段需要重复存储，部门名字越长，越浪费。

这个时候，

解决方法：

我们完全可以定义一个部门表

然后让员工信息表关联该表，如何关联，即foreign key

我们可以将上表改成如下结构

此时有两张表，一张是employee表（关联表，也就是从表）。一张是department表，简称dep表（被惯量表，也叫主表）。

创建两张表操作：

#1.创建表时先创建被关联表，再创建关联表# 先创建被关联表（dep表）create table dep(id int primary key,name varchar(20) not null,descripe varchar(20) not null);#再创建关联表（emp表）create table emp(id int primary key,name varchar(20) not null,age int not null,dep_id int,constraint fk_dep foreign key(dep_id) references dep(id) );#2.插入记录时，先往被关联表中插入记录，再往关联表中插入记录insert into dep values(1,'IT','IT技术有限部门'),(2,'销售部','销售部门'),(3,'财务部','花钱太多部门');insert into emp values(1,'zhangsan',18,1),(2,'lisi',19,1),(3,'egon',20,2),(4,'yuanhao',40,3),(5,'alex',18,2);3.删除表#按道理来说，删除了部门表中的某个部门，员工表的有关联的记录相继删除。mysql> delete from dep where id=3;ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`db5`.`emp`, CONSTRAINT `fk_name` FOREIGN KEY (`dep_id`) REFERENCES `dep` (`id`))#但是先删除员工表的记录之后，再删除当前部门就没有任何问题mysql> delete from emp where dep =3;Query OK, 1 row affected (0.00 sec)mysql> select * from emp;+----+----------+-----+--------+| id | name | age | dep_id |+----+----------+-----+--------+|  1 | zhangsan |  18 |  1 ||  2 | lisi |  18 |  1 ||  3 | egon |  20 |  2 ||  5 | alex |  18 |  2 |+----+----------+-----+--------+4 rows in set (0.00 sec)mysql> delete from dep where id=3;Query OK, 1 row affected (0.00 sec)mysql> select * from dep;+----+-----------+----------------------+| id | name  | descripe |+----+-----------+----------------------+|  1 | IT| IT技术有限部门||  2 | 销售部| 销售部门 |+----+-----------+----------------------+2 rows in set (0.00 sec)

上面的删除表记录的操作比较繁琐，按道理讲，裁掉一个部门，该部门的员工也会被裁掉，其实呢，在建表的时候还有个很重要的内容，叫同步删除，同步更新

接下来将刚建好的的两张表全部删除，先删除关联表（emp），在删除被关联表（dep）

接下来：

重复上面的操作建表

注意：在关连表中加入

on delete cascade  #同步删除

on update cascade  #同步更新

修改emp表：

create table emp(id int primary key,name varchar(20) not null,age int not null,dep_id int,constraint fk_dep foreign key(dep_id) references dep(id) on delete cascade #同步删除on update cascade #同步更新);

接下来的操作，就符合我们正常的生活中的情况了。

#再去删被关联表(dep)的记录，关联表(emp)中的记录也跟着删除mysql> delete from dep where id=3;Query OK, 1 row affected (0.00 sec)mysql> select * from dep;+----+-----------+----------------------+| id | name  | descripe |+----+-----------+----------------------+|  1 | IT| IT技术有限部门||  2 | 销售部| 销售部门 |+----+-----------+----------------------+2 rows in set (0.00 sec)mysql> select * from emp;+----+----------+-----+--------+| id | name | age | dep_id |+----+----------+-----+--------+|  1 | zhangsan |  18 |  1 ||  2 | lisi |  19 |  1 ||  3 | egon |  20 |  2 ||  5 | alex |  18 |  2 |+----+----------+-----+--------+4 rows in set (0.00 sec)#再去更改被关联表(dep)的记录，关联表(emp)中的记录也跟着更改mysql> update dep set id=222 where id=2;Query OK, 1 row affected (0.02 sec)Rows matched: 1  Changed: 1  Waings: 0# 赶紧去查看一下两张表是否都被删除了，是否都被更改了mysql> select * from dep;+-----+-----------+----------------------+| id  | name  | descripe |+-----+-----------+----------------------+|1 | IT| IT技术有限部门|| 222 | 销售部| 销售部门 |+-----+-----------+----------------------+2 rows in set (0.00 sec)mysql> select * from emp;+----+----------+-----+--------+| id | name | age | dep_id |+----+----------+-----+--------+|  1 | zhangsan |  18 |  1 ||  2 | lisi |  19 |  1 ||  3 | egon |  20 |222 ||  5 | alex |  18 |222 |+----+----------+-----+--------+4 rows in set (0.00 sec)