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python/MySQL入门/9-MySQL表的完整性约束

2020-10-22

一、概览

为了防止不符合规范的数据进入数据库,在用户对数据进行插入、修改、删除等操作时,DBMS自动按照一定的约束条件对数据进行监测,使不符合规范的数据不能进入数据库,以确保数据库中存储的数据正确、有效、相容。

约束条件与数据类型的宽度一样,都是可选参数,主要分为以下几种:

约束条件 解释
NOT NULL 非空约束,指定某列不能为空;
UNIQUE 唯一约束,指定某列或者几列组合不能重复
PRIMARY KEY 主键,指定该列的值可以唯一地标识该列记录
FOREIGN KEY 外键,指定该行记录从属于主表中的一条记录,主要用于参照完整性

二、NOT NULL

是否可空,null表示空,非字符串

not null - 不可空

null - 可空

2.1 not null实例

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mysql> create table t12 (id int not null);
Query OK, 0 rows affected (0.02 sec)

mysql> select * from t12;
Empty set (0.00 sec)

mysql> desc t12;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
+-------+---------+------+-----+---------+-------+
row in set (0.00 sec)

#不能向id列插入空元素。
mysql> insert into t12 values (null);
ERROR 1048 (23000): Column 'id' cannot be null

mysql> insert into t12 values (1);
Query OK, 1 row affected (0.01 sec)

2.2 DEFAULT

我们约束某一列不为空,如果这一列中经常有重复的内容,就需要我们频繁的插入,这样会给我们的操作带来新的负担,于是就出现了默认值的概念。

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。

2.3 not null + default 示例

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mysql> create table t13 (id1 int not null,id2 int not null default 222);
Query OK, 0 rows affected (0.01 sec)

mysql> desc t13;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id1 | int(11) | NO | | NULL | |
| id2 | int(11) | NO | | 222 | |
+-------+---------+------+-----+---------+-------+
rows in set (0.01 sec)

# 只向id1字段添加值,会发现id2字段会使用默认值填充
mysql> insert into t13 (id1) values (111);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
+-----+-----+
row in set (0.00 sec)

# id1字段不能为空,所以不能单独向id2字段填充值;
mysql> insert into t13 (id2) values (223);
ERROR 1364 (HY000): Field 'id1' doesn't have a default value

# 向id1,id2中分别填充数据,id2的填充数据会覆盖默认值
mysql> insert into t13 (id1,id2) values (112,223);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
| 112 | 223 |
+-----+-----+
rows in set (0.00 sec)

2.4 not null不生效

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# 设置严格模式:
不支持对not null字段插入null值
不支持对自增长字段插入”值
不支持text字段有默认值

# 直接在mysql中生效(重启失效):
mysql>set sql_mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION";

# 配置文件添加(永久失效):
sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"

三、UNIQUE

唯一约束,指定某列或者几列组合不能重复。

3.1 unique示例

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# 方法一:
create table department1(
id int,
name varchar(20) unique,
comment varchar(100)
);


# 方法二:
create table department2(
id int,
name varchar(20),
comment varchar(100),
unique(name)
);


mysql> insert into department1 values(1,'IT','技术');
Query OK, 1 row affected (0.00 sec)
mysql> insert into department1 values(1,'IT','技术');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

3.2 not null 和 unique 的结合

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mysql> create table t1(id int not null unique);
Query OK, 0 rows affected (0.02 sec)

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
+-------+---------+------+-----+---------+-------+
row in set (0.00 sec)

3.3 联合唯一

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create table service(
id int primary key auto_increment,
name varchar(20),
host varchar(15) not null,
port int not null,
unique(host,port) #联合唯一
);

mysql> insert into service values
-> (1,'nginx','192.168.0.10',80),
-> (2,'haproxy','192.168.0.20',80),
-> (3,'mysql','192.168.0.30',3306)
-> ;
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> insert into service(name,host,port) values('nginx','192.168.0.10',80);
ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'

四、PRIMARY KEY

主键为了保证表中的每一条数据的该字段都是表格中的唯一值。换言之,它是用来独一无二地确认一个表格中的每一行数据。
主键可以包含一个字段或多个字段。当主键包含多个栏位时,称为组合键 (Composite Key),也可以叫联合主键。
主键可以在建置新表格时设定 (运用 CREATE TABLE 语句),或是以改变现有的表格架构方式设定 (运用 ALTER TABLE)。
主键必须唯一,主键值非空;可以是单一字段,也可以是多字段组合。

4.1 单字段主键

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============单列做主键===============
#方法一:not null+unique
create table department1(
id int not null unique, #主键
name varchar(20) not null unique,
comment varchar(100)
);

mysql> desc department1;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | NO | UNI | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

#方法二:在某一个字段后用primary key
create table department2(
id int primary key, #主键
name varchar(20),
comment varchar(100)
);

mysql> desc department2;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.00 sec)

#方法三:在所有字段后单独定义primary key
create table department3(
id int,
name varchar(20),
comment varchar(100),
primary key(id); #创建主键并为其命名pk_name

mysql> desc department3;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

# 方法四:给已经建成的表添加主键约束
mysql> create table department4(
-> id int,
-> name varchar(20),
-> comment varchar(100));
Query OK, 0 rows affected (0.01 sec)

mysql> desc department4;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

mysql> alter table department4 modify id int primary key;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc department4;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

4.2 多字段主键

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==================多列做主键================
create table service(
ip varchar(15),
port char(5),
service_name varchar(10) not null,
primary key(ip,port)
);


mysql> desc service;
+--------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip | varchar(15) | NO | PRI | NULL | |
| port | char(5) | NO | PRI | NULL | |
| service_name | varchar(10) | NO | | NULL | |
+--------------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

mysql> insert into service values
-> ('172.16.45.10','3306','mysqld'),
-> ('172.16.45.11','3306','mariadb')
-> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> insert into service values ('172.16.45.10','3306','nginx');
ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'

4.3 AUTO_INCREMENT

约束字段为自动增长,被约束的字段必须同时被key约束

4.3.1 设置auto_increment

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#不指定id,则自动增长
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 | |
+-------+-----------------------+------+-----+---------+----------------+
mysql> insert into student(name) values
-> ('lqz'),
-> ('tank')
-> ;

mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | lqz | male |
| 2 | tank | male |
+----+------+------+


#也可以指定id
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 (0.00 sec)

mysql> select * from student;
+----+------+--------+
| id | name | sex |
+----+------+--------+
| 1 | lqz | male |
| 2 | tank | male |
| 4 | asb | female |
| 7 | wsb | female |
+----+------+--------+


#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, 4 rows affected (0.00 sec)

mysql> select * from student;
Empty set (0.00 sec)

mysql> insert into student(name) values('ysb');
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 8 | ysb | male |
+----+------+------+

#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 (0.01 sec)

mysql> insert into student(name) values('lqz');
Query OK, 1 row affected (0.01 sec)

mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | lqz | male |
+----+------+------+
row in set (0.00 sec)

4.4 offset偏移量(了解)

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#在创建完表后,修改自增字段的起始值
mysql> create table student(
-> id int primary key auto_increment,
-> name varchar(20),
-> sex enum('male','female') default 'male'
-> );

mysql> alter table student auto_increment=3;

mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

mysql> insert into student(name) values('lqz');
Query OK, 1 row affected (0.01 sec)

mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 3 | lqz | male |
+----+------+------+
row in set (0.00 sec)

mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8


#也可以创建表时指定auto_increment的初始值,注意初始值的设置为表选项,应该放到括号外
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
)auto_increment=3;




#设置步长
sqlserver:自增步长
基于表级别
create table t1(
id int。。。
)engine=innodb,auto_increment=2 步长=2 default charset=utf8

mysql自增的步长:
show session variables like 'auto_inc%';

#基于会话级别
set session auth_increment_increment=2 #修改会话级别的步长

#基于全局级别的
set global auth_increment_increment=2 #修改全局级别的步长(所有会话都生效)


#!!!注意了注意了注意了!!!
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 ,这相当于第一步步子就迈大了,扯着了蛋
比如:设置auto_increment_offset=3,auto_increment_increment=2




mysql> set global auto_increment_increment=5;
Query OK, 0 rows affected (0.00 sec)

mysql> set global auto_increment_offset=3;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'auto_incre%'; #需要退出重新登录
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+



create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
);

mysql> insert into student(name) values('lqz1'),('lqz2'),('lqz3');
mysql> select * from student;
+----+-------+------+
| id | name | sex |
+----+-------+------+
| 3 | lqz1 | male |
| 8 | lqz2 | male |
| 13 | lqz3 | male |
+----+-------+------+

步长:auto_increment_increment,起始偏移量:auto_increment_offset

五、FOREIGN KEY

多表 :

假设我们要描述所有公司的员工,需要描述的属性有这些 : 工号 姓名 部门

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

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

5.1 创造外键的条件

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mysql> create table departments (dep_id int(4),dep_name varchar(11));
Query OK, 0 rows affected (0.02 sec)

mysql> desc departments;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id | int(4) | YES | | NULL | |
| dep_name | varchar(11) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

# 创建外键不成功
mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id));
ERROR 1215 (HY000): Cannot add foreign key

# 设置dep_id非空,仍然不能成功创建外键
mysql> alter table departments modify dep_id int(4) not null;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc departments;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id | int(4) | NO | | NULL | |
| dep_name | varchar(11) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id));
ERROR 1215 (HY000): Cannot add foreign key constraint

# 当设置字段为unique唯一字段时,设置该字段为外键成功
mysql> alter table departments modify dep_id int(4) unique;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc departments; +----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id | int(4) | YES | UNI | NULL | |
| dep_name | varchar(11) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
rows in set (0.01 sec)

mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id));
Query OK, 0 rows affected (0.02 sec)

5.2 外键操作示例

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#表类型必须是innodb存储引擎,且被关联的字段,即references指定的另外一个表的字段,必须保证唯一
create table department(
id int primary key,
name varchar(20) not null
)engine=innodb;

#dpt_id外键,关联父表(department主键id),同步更新,同步删除
create table employee(
id int primary key,
name varchar(20) not null,
dpt_id int,
foreign key(dpt_id)
references department(id)
on delete cascade # 级连删除
on update cascade # 级连更新
)engine=innodb;


#先往父表department中插入记录
insert into department values
(1,'教质部'),
(2,'技术部'),
(3,'人力资源部');


#再往子表employee中插入记录
insert into employee values
(1,'jason',1),
(2,'oscar',2),
(3,'lqz',2),
(4,'tank',2),
(5,'mac',3),
(6,'李沁洋',3),
(7,'皮卡丘',3),
(8,'程咬金',3),
(9,'程咬银',3)
;


#删父表department,子表employee中对应的记录跟着删
mysql> delete from department where id=2;
Query OK, 1 row affected (0.00 sec)

mysql> select * from employee;
+----+-----------+--------+
| id | name | dpt_id |
+----+-----------+--------+
| 1 | jason | 1 |
| 5 | mac | 3 |
| 6 | 李沁洋 | 3 |
| 7 | 皮卡丘 | 3 |
| 8 | 程咬金 | 3 |
| 9 | 程咬银 | 3 |
+----+-----------+--------+
rows in set (0.00 sec)


#更新父表department,子表employee中对应的记录跟着改
mysql> update department set id=2 where id=3;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from employee;
+----+-----------+--------+
| id | name | dpt_id |
+----+-----------+--------+
| 1 | jason | 1 |
| 5 | mac | 2 |
| 6 | 李沁洋 | 2 |
| 7 | 皮卡丘 | 2 |
| 8 | 程咬金 | 2 |
| 9 | 程咬银 | 2 |
+----+-----------+--------+
rows in set (0.00 sec)

5.3 on delete(了解)

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#  cascade方式
在父表上update/delete记录时,同步update/delete掉子表的匹配记录

# set null方式
在父表上update/delete记录时,将子表上匹配记录的列设为null
要注意子表的外键列不能为not null

# No action方式
如果子表中有匹配的记录,则不允许对父表对应候选键进行update/delete操作

# Restrict方式
同no action, 都是立即检查外键约束

# Set default方式
父表有变更时,子表将外键列设置成一个默认的值 但Innodb不能识别
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