Detailed introduction to mysql integrity constraints

1. Introduction

The constraints are the same as the width of the data type and are optional parameters

Function: Used to ensure data The integrity and consistency

is mainly divided into:

PRIMARY KEY (PK) #Identifies this field as the primary key of the table, which can be uniquely identified Record

FOREIGN KEY (FK) #Identifies this field as the foreign key of the table

NOT NULL #Identifies that this field cannot be empty

UNIQUE KEY (UK) #The value that identifies this field is unique

AUTO_INCREMENT #The value that identifies this field grows automatically (integer type, And it is the primary key)

DEFAULT #Set the default value for this field

UNSIGNED #Unsigned

ZEROFILL #Use 0 to fill

Instructions:

#1. Whether to allow empty, the default is NULL, NOT NULL can be set, the field is not allowed Is empty, must be assigned a value

#2. Whether the field has a default value, the default value is NULL, if the field is not assigned a value when inserting the record, this field uses the default value

sex enum ('male','female') not null default 'male'

#Must be a positive value (unsigned), not allowed to be empty, the default is 20age int unsigned NOT NULL default 20

# 3. Whether it is key

primary key

foreign key foreign key

index (index, unique...)

2. NOT NULL and DEFAULT

Whether it is nullable, null means empty, non-string
not null - not nullable
null - nullable

Default value, you can specify the default value when creating a column. If it is not actively set when inserting data, the default value will be automatically added

create table tb1(
    nid int not null defalut 2,    
    num int not null);

Note:

1. Default The value can be empty

2. Set not null, and it cannot be empty when inserting the value

3. After setting the id field to have a default value, it will work regardless of whether the id field is null or not null. Insert a blank, and the default value specified by default is filled in when inserting a blank

3. UNIQUE

Chinese translation: different. In mysql, it is called single column unique

Example: Create a company department table (each company has a unique department)

mysql> create table department(
    -> id int,
    -> name char(10)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into department values(1,'IT'),(2,'IT');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0mysql> select * from department;
+------+------+| id   | name |
+------+------+|    1 | IT   |
|    2 | IT   |
+------+------+2 rows in set (0.00 sec)
# 发现： 同时插入两个IT部门也是可以的，但这是不合理的，所以我们要设置name字段为unique 解决这种不合理的现象。

Next, use the constraint unique to define the company department fields to set.

#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');

United uniqueness:

# 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'

4. PRIMARY KEY

There is only one unique in a table in MySQL A primary key cannot have multiple columns, but it can have a composite primary key

A table can have:

Single column as the primary key
Multiple columns as the primary key (composite primary key)

Constraint: Equivalent to not null unique, the value of the field is not empty and unique

The storage engine default is (innodb): For the innodb storage engine, a table must have a primary key.

Single column primary key:

# 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 key
create 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)

Composite primary key:

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);

5. AUTO_INCREMENT

Constraints: The constrained fields are automatically grown, and the constrained fields must be constrained by key at the same time

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

1. If the id is not specified, it will grow automatically

2. You can also specify the id

3. For a self-increasing field, after deleting it with delete and then inserting the value, the field will continue to grow according to the position before deletion

##auto_increment_increment and auto_increment_offset The difference between

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 |
+----+---------+------+
auto_increment_increment和 auto_increment_offset

Clear table distinction between

delete and truncate:

delete from t1; #If there is an auto-increment id, the new one The data still starts from the last one before deletion.

truncate table t1;The amount of data is large, the deletion speed is faster than the previous one, and it starts directly from zero.

6. FOREIGN KEY

The company has 3 departments, but there are 100 million employees, which means that the department field needs to be stored repeatedly. The longer the department name, the more wasteful it is.

At this time,

Solution:

We can completely define a department table

And then let the employee information table associate with the table, how to associate, that is foreign key

Create two tables operation:

#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,'djb',20,2),
(4,'dogfa',40,3),
(5,'oldniu',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 | djb      |  20 |      2 |
|  5 | oldniu   |  18 |      2 |
+----+----------+-----+--------+
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             |
+----+-----------+----------------------+
rows in set (0.00 sec)

The above operation of deleting table records is relatively cumbersome. Logically speaking, if a department is laid off, the employees in that department will also be laid off. In fact, there is another very important content when creating a table, which is called synchronous deletion and synchronous update

Next, delete all the two newly created tables. First delete the associated table (emp), and then Delete the related table (dep)

Next:

Repeat the above operation to create the table

Note: Add
on delete cascade to the related table #Synchronous deletion
on update cascade #Synchronous update

Modify emp table:

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
);

接下来的操作，就符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             |
+----+-----------+----------------------+
rows in set (0.00 sec)
mysql> select * from emp;
+----+----------+-----+--------+
| id | name     | age | dep_id |
+----+----------+-----+--------+
|  1 | zhangsan |  18 |      1 |
|  2 | lisi     |  19 |      1 |
|  3 | djb      |  20 |      2 |
|  5 | oldniu   |  18 |      2 |
+----+----------+-----+--------+
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             |
+-----+-----------+----------------------+
rows in set (0.00 sec)
mysql> select * from emp;
+----+----------+-----+--------+
| id | name     | age | dep_id |
+----+----------+-----+--------+
|  1 | zhangsan |  18 |      1 |
|  2 | lisi     |  19 |      1 |
|  3 | djb      |  20 |    222 |
|  5 | oldniu   |  18 |    222 |
+----+----------+-----+--------+
rows in set (0.00 sec)

以上便是常用约束的详细实例介绍，想了解更多相关问题请访问PHP中文网：mysql视频教程

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