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完整性约束
阅读量:6280 次
发布时间:2019-06-22

本文共 14828 字,大约阅读时间需要 49 分钟。

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:

RIMARY 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

说明:

#1. 是否允许为空,默认NULL,可设置NOT NULL,字段不允许为空,必须赋值#2. 字段是否有默认值,缺省的默认值是NULL,如果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 默认是20age int unsigned NOT NULL default 20 # 3. 是否是key主键 primary key外键 foreign key索引 (index,unique...)

二、not null 与default

是否可空,null表示空,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 defalut 2,    num int not null);

 

验证1:

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)
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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
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验证3:

第一种情况mysql> create table t13(id int default 1);Query OK, 0 rows affected (0.03 sec)mysql> desc t13;+-------+---------+------+-----+---------+-------+| Field | Type    | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| id    | int(11) | YES  |           | 1              |              |+-------+---------+------+-----+---------+-------+1 row in set (0.01 sec)mysql> insert into t13 values();Query OK, 1 row affected (0.00 sec)mysql> select * from t13;+------+| id   |+------+|    1  |+------+1 row in set (0.00 sec)# 第二种情况mysql> create table t14(id int not null default 2);Query OK, 0 rows affected (0.02 sec)mysql> desc t14;+-------+---------+------+-----+---------+-------+| Field | Type    | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| id    | int(11) | NO      |         | 2               |             |+-------+---------+------+-----+---------+-------+1 row in set (0.01 sec)mysql> select * from t14;+----+| id |+----+|  2 |+----+1 row in set (0.00 sec)
设置id字段有默认值后,则无论id字段是null还是not null,都可以插入空,插入空默认填入default指定的默认值

小练习:

创建学生表student2,设置每个字段的约束条件。

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)
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3、unique

中文翻译:不同的。在mysql中称为单列唯一

 

举例说明:创建公司部门表(每个公司都有唯一的一个部门)。

mysql> create table department(    -> id int,    -> name char(10)    -> );Query OK, 0 rows affected (0.01 sec
验证之前重复插入记录的操作是可行的,但是不符合场景

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  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'
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4.primary key

一个表中可以:

单列做主键

多列做主键(复合主键)

约束:等价于 not null unique,字段的值不为空且唯一

存储引擎默认是(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);
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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  Warnings: 0mysql> select * from student;+----+--------+------+| id | name   | sex  |+----+--------+------+|  1 | 老白   | male ||  2 | 小白   | male |+----+--------+------+2 rows in set (0.00 sec)
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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 |+----+--------+--------+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)mysql>

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  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。

6.foreign key

 

一 快速理解foreign key

之前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3个部门,但是有1个亿的员工,那意味着部门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

我们可以将上表改为如下结构:

 

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)。

 

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)
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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

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

接下来:

重复上面的操作建表
注意:在关联表中加入
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);
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接下来的操作,就复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  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)

 

转载于:https://www.cnblogs.com/Robi-9662/p/9416666.html

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