mysql完整性约束实例详解

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

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

#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

# 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: 0
mysql> 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

# 创建student
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 | |
+-------+-----------------------+------+-----+---------+----------------+
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: 0
mysql> 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

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

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

mysql> show variables like 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+
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 |
+--------------------------+-------+
rows in set (0.00 sec)
#因为之前有一条记录id=1
mysql> 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+5
mysql> insert into student(name) values('ma1'),('ma2'),('ma3');
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> 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）。

#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

#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

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
);

#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)

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