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    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

    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。

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