本文实例讲述了mysql完整性约束。分享给大家供大家参考，具体如下：
主要内容
- not null 与 default
- unique
- primary
- auto_increment
- foreign key
约束条件作用：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
unique
在mysql中称为单列唯一
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#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：只要两列记录，有一列不同，既符合联合唯一的约束
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# 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 Warnings: 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' |
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
不指定id，则自动增长
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# 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 | |+-------+-----------------------+------+-----+---------+----------------+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 Warnings: 0mysql> select * from student;+----+--------+------+| id | name | sex |+----+--------+------+| 1 | 老白 | male || 2 | 小白 | male |+----+--------+------+rows in set (0.00 sec) |
指定id的情况
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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 |+----+--------+--------+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 |+----+--------+--------+rows in set (0.00 sec) |
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
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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 |+----+------+------+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 |+----+---------+------+row in set (0.00 sec)mysql>auto_increment_increment和 auto_increment_offset |
查看可用的 开头auto_inc的词
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mysql> show variables like 'auto_inc%';+--------------------------+-------+| Variable_name | Value |+--------------------------+-------+| auto_increment_increment | 1 || auto_increment_offset | 1 |+--------------------------+-------+rows in set (0.02 sec) |
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# 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。
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mysql> show variables like'auto_inc%';+--------------------------+-------+| Variable_name | Value |+--------------------------+-------+| auto_increment_increment | 5 || auto_increment_offset | 3 |+--------------------------+-------+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 |+----+---------+------+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 Warnings: 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。
foreign key
理解foreign key
如上图如果一个公司有很多员工，每个员工都对应一个部门，在填表的时候就会重复写这些部门，太冗余了
我们可以将它们分离
此时有两张表，一张是employee表,简称emp表（关联表，也就从表）。一张是department表，简称dep表（被关联表，也叫主表）。
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#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) //创建约束);#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
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#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_id =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
on delete cascade #同步删除
on update cascade #同步更新
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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); |
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#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 Warnings: 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) |
希望本文所述对大家MySQL数据库计有所帮助。
原文链接：https://www.cnblogs.com/mmyy-blog/p/9626793.html
