查看: 42|回复: 0
上一主题 下一主题

mysql完整性约束实例详解

78058

主题

0

好友

积分

离线 发信

跳转到指定楼层
楼主
发表于 2020-01-26 21:38 | 只看该作者 | 倒序浏览

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)

更多关于MySQL相关内容感兴趣的读者可查看本站专题:《MySQL查询技巧大全》、《MySQL常用函数大汇总》、《MySQL日志操作技巧大全》、《MySQL事务操作技巧汇总》、《MySQL存储过程技巧大全》及《MySQL数据库锁相关技巧汇总》

希望本文所述对大家MySQL数据库计有所帮助。

来源:http://www.jb51.net/article/177768.htm