[MySQL] Table constraints

One . What are constraints ？

When I introduced data types in the previous article, I introduced “ constraint ” The concept , What really constrains the field is the data type , But the data type constraint is very single , There need to be some extra constraints , Better guarantee the legitimacy of the data , This article will focus on MySQL Some of the constraints in .

The definition of constraints is given below ： Constraints are essentially MySQL By limiting user actions , To achieve a set of integrity solutions to maintain its own security , That is, the ultimate goal of constraints is to ensure the correctness of data from the perspective of business logic 、 Security .

Two . Why should there be constraints ？

We know MySQL Is a set of data storage solutions , In addition to the basic data storage functions , Also ensure that the data is as secure as possible , Reduce the possibility of user misoperation .

It's like 《 Trisomy 》 Facing the wall plan in , Now that the gravitational wave deterrent system has been built , That's it , Why choose a man who faces the wall （ Swordsman ） Well ？ Because people all over the world need to ensure the safety of gravitational wave deterrence system （ That is, you can't use it at will ）, Therefore, it is necessary to select a unified candidate to control the system , To maintain the strategic balance between the two civilizations , The sword bearer is a restraint .

3、 ... and . How to constrain ？

1. Empty properties — not null

When we register the account or fill in the user information , Some properties are required 、 The other is optional ：

Corresponding to the database level , When we define each field in the table , They have two properties ：null（ default ） and not null( Not empty ). The default field properties of the database are allowed to be empty , But in actual development , Try to make sure that the field is not empty , Because the data is empty .

• Create a Person surface , The field contains the name of the person 、 Gender , All allowed to be empty ：
  
• Create a PersonNotNull surface , It also contains two fields ： full name 、 Gender . However, it is not allowed to be empty ：
  

2. The default value is — default

Usually when we register a social platform account , Need to bind your phone number , Different countries have different international area codes for telephone numbers ; For example, China's international area code is +86, If this social media platform is Chinese , The default is that the Chinese use the most , So if you don't choose, the area code is +86 了 ：

The default value is ： A certain kind of data will often have a specific value , You can specify at the beginning , When you need real data , Users can selectively use the default value .

Usage method ： When defining fields, add default The default value is .

give an example ：

mysql> create table if not exists Person(
    -> name varchar(20),
    -> nationality varchar(20) default ' China ',
    -> age tinyint unsigned default 0
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> desc Person;//  The nationality and age fields use default values 
+-------------+---------------------+------+-----+---------+-------+
| Field       | Type                | Null | Key | Default | Extra |
+-------------+---------------------+------+-----+---------+-------+
| name        | varchar(20)         | YES  |     | NULL    |       |
| nationality | varchar(20)         | YES  |     |  China     |       |
| age         | tinyint(3) unsigned | YES  |     | 0       |       |
+-------------+---------------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> insert into Person (name) values (' Zhang San ');
Query OK, 1 row affected (0.00 sec)

mysql> select * from Person;
+--------+-------------+------+
| name   | nationality | age  |
+--------+-------------+------+
|  Zhang San    |  China         |    0 |
+--------+-------------+------+
1 row in set (0.00 sec)

mysql>

problem ：not null and default Simultaneous setting , What will happen ？

The following code tests this problem . When we build tables , Give Way nationality The field is not null Also set the default value default; Found that when inserting data , No error will be reported even if this field is not filled in , And it is found that the default value of this field is ：

mysql> create table if not exists Person(
    -> nationality varchar(20) not null default ' China ',
    -> age tinyint unsigned
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> insert into Person (age) values (18);
Query OK, 1 row affected (0.01 sec)

mysql> select * from Person;
+-------------+------+
| nationality | age  |
+-------------+------+
|  China         |   18 |
+-------------+------+
1 row in set (0.00 sec)

mysql>

summary ： stay not null and default At the same time , You can still use the default value without filling in the value , No mistake. . But it is not recommended to use it together , In the fields that must be filled with data, only not null, Only use... When the default value is determined in advance default.

3. Columns. — comment

Columns. (comment), There's no real meaning , Specifically used to describe field information , The function is equivalent to annotation . It will create statements based on the table to save , For programmers or DBA To understand .

View by ： Column description information is in use desc Keyword will not be displayed when viewing the table structure , Only through show When viewing the table creation statement ：

mysql> create table if not exists Person(
    -> name varchar(20) comment ' This is the user's name , Required ',
    -> gender char(1) default ' male ' comment ' This is the gender of the user , The default is male ',
    -> age tinyint unsigned default 18 comment ' This is the age of the user , The default is 18'
    -> );
Query OK, 0 rows affected (0.02 sec)

// 1、 adopt desc Unable to view comment information 
mysql> desc Person;
+--------+---------------------+------+-----+---------+-------+
| Field  | Type                | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| name   | varchar(20)         | YES  |     | NULL    |       |
| gender | char(1)             | YES  |     |  male       |       |
| age    | tinyint(3) unsigned | YES  |     | 18      |       |
+--------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

// 2、 adopt show When you view the table creation statement, you can see the annotation information 
mysql> show create table Person \G
*************************** 1. row ***************************
       Table: Person
Create Table: CREATE TABLE `Person` (
  `name` varchar(20) DEFAULT NULL COMMENT ' This is the user's name , Required ',
  `gender` char(1) DEFAULT ' male ' COMMENT ' This is the gender of the user , The default is male ',
  `age` tinyint(3) unsigned DEFAULT '18' COMMENT ' This is the age of the user , The default is 18'
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql>

Use advice ： The class description is best written in the last position of the field , Because it's not like not null or default That will have a strong impact on the table data , In this way, it is easier for us to interpret the information of this field .

4. zerofill

up to now , We learned that the following two types can be followed by parentheses ：

• Character type ：char(len)、varchar(len). The number of parentheses here determines the upper limit of the number of stored characters .
• value type ：int(int)、tinyint(int) etc. . The number in parentheses here determines the bit width of the displayed data , Just add zerofill Keywords are useful .

give an example ：
Let's create a table , There are two of them int Type field a and b, It is stipulated that their bit widths are 10, Only b Field added keywords zerofill, Insert a piece of data into the table , Observe the display result after insertion ：

mysql> create table if not exists Number(
    -> a int(10),
    -> b int(10) zerofill
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> desc Number;
+-------+---------------------------+------+-----+---------+-------+
| Field | Type                      | Null | Key | Default | Extra |
+-------+---------------------------+------+-----+---------+-------+
| a     | int(10)                   | YES  |     | NULL    |       |
| b     | int(10) unsigned zerofill | YES  |     | NULL    |       |
+-------+---------------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into Number values (10, 20);
Query OK, 1 row affected (0.00 sec)

mysql> select * from Number;
+------+------------+
| a    | b          |
+------+------------+
|   10 | 0000000020 |
+------+------------+
1 row in set (0.00 sec)

mysql>

You can see b The value of shows that the bit width is 10, and a The value of is the normal display . This is it. zerofill The function of attributes , If the width of the data itself is less than the set width （ The setting here is 10）, Will be automatically filled 0 The set bit width has been reached . It should be noted that , It's just the last result , stay MySQL What is actually stored in the is 20. Why is that so ？ We can use hex Function to prove , The function is based on 16 Display the value in decimal form ：

mysql> select b, hex(b) from Number;
+------------+--------+
| b          | hex(b) |
+------------+--------+
| 0000000020 | 14     |
+------------+--------+
1 row in set (0.00 sec)

mysql>

It can be seen that the internal storage of the database is still digital 20,0000000020 Just set up zerofill Property is just a formatted output .

5. Primary key — primary key

Primary key (primary key) Is a field that uniquely identifies a row of data in a table . If you compare a watch to a red black tree , Each row of data in the table corresponds to each node of the red black tree ; The primary key of the table corresponds to... In the node key; The other fields of the table correspond to value.

The primary key of a table has the following three characteristics ：

• A table has only one primary key field , This field uniquely identifies a row of data in the table .
• The default primary key field is not null attribute .
• The contents of the primary key field in each row of the table cannot be repeated .

give an example ：

Composite main building

Create a Address surface , hold ip Fields and port Fields are combined to form a composite primary key , It is used to identify the only process in the public network ：

mysql> create table if not exists Address(
    -> ip varchar(20) comment ' Host's public network IP',
    -> port int unsigned comment ' Unique identification of the process in the host ',
    -> name varchar(20) comment ' Host username ',
    -> primary key(ip, port)
    -> );
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> desc Address;
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| ip    | varchar(20)      | NO   | PRI | NULL    |       |
| port  | int(10) unsigned | NO   | PRI | NULL    |       |
| name  | varchar(20)      | YES  |     | NULL    |       |
+-------+------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql>

Only when the contents of all fields in the composite primary key are identical , Primary key conflicts ：

mysql> insert into Address values ('127.0.1', 8080, ' Zhang San ');
Query OK, 1 row affected (0.01 sec)

mysql> insert into Address values ('127.0.1', 1111, ' Zhang San ');
Query OK, 1 row affected (0.17 sec)

mysql> select * from Address;
+---------+------+--------+
| ip      | port | name   |
+---------+------+--------+
| 127.0.1 | 1111 |  Zhang San    |
| 127.0.1 | 8080 |  Zhang San    |
+---------+------+--------+
2 rows in set (0.00 sec)

mysql>

Primary key modification

1、 Append primary key
When the table is created but there is no primary key , You can append the primary key again with the following statement , However, it is necessary to ensure the uniqueness of the data of the field appended as the primary key ：

alter table  Table name  add primary key( Field list );

2、 Delete primary key
There is only one primary key in a table , Therefore, you do not need to specify specific fields when deleting a primary key ：

alter table  Table name  drop primary key;

Suggestions for setting table primary key

generally speaking , We suggest that the primary key be designed as Fields unrelated to the current business , such , When the business is adjusted , We can try not to adjust the primary key too much .

6. Self growth — auto_increment

auto_increment： Fields set as self growing must be used with the primary key , As a logical primary key . This field may not be given a value , It will be automatically triggered by the system , From the self growth field of the current table The maximum value already exists Conduct +1 operation , Get a new different value .

The characteristics of self growth ：

• The self growing field must be of type integer .
• The self growing field must be a field in the primary key or composite primary key .
• A table can only have one self growing field at most .

Usage method ： When the table is created , Add... After the field auto_increment; If it's a composite primary key , You need to put the self growing column in the first position of the composite primary key , On the far left , As shown in the following code ：

//  Put the... Under the composite main construction a The field is set to self growth 
//  If you insert it like this , You only need to specify non self growing fields 
create table t(
	a int auto_increment,
	b int,
	primary key (a,b)
);

give an example ：
Create a student table , Set the student ID as self increasing , In this way, each time you insert a row of data, you only need to specify the student name , Their student numbers are allocated according to the value given by self growth ：

There is no row of data in the current table , We insert a student named “ Zhang San ”, The student ID uses the value given by self growth , It is found that the default value is a number at the beginning of the growth 1：

Continue inserting data , It is observed that the given value of self growth is based on the existing maximum value +1：

In order to verify the law of self growth again , This time we insert our own “ Student number , full name ” data , Then observe the change of self growth value ：

7. The only key — unique

There are many fields in a table that need uniqueness , That is, the data of this field in a table cannot be repeated , But a table can only have one primary key , What about other fields that need to be unique ？ A unique key can solve the problem that multiple fields in a table need uniqueness constraints .

Like at school , We need a student management system , There is a student list in the system , There are two fields in the student table , An ID number , One is the student number , We can choose to use the ID number as the primary key . So the student ID is just a common field ,MySQL A security constraint is not provided for it , In case you accidentally input a duplicate student ID, it will not be detected , It's very troublesome not to change until there is something wrong with the business . So when we design student numbers , Need a constraint ： All student numbers cannot be repeated , We can design the student ID as the unique key when designing the table .

Unique key setting method ： Add... When defining fields unique keyword .

The difference between a unique key and a primary key ：

• The unique key is allowed to be empty , And more than one can be empty , That is, empty fields are not used as the basis for uniqueness .
• The main point of a primary key is that it serves as a unique identifier for a row of data ; The only key focuses on ensuring the uniqueness of the field data in the table .

give an example ：
Create a learning information table , Set the student ID as the unique key and verify the unique key characteristics through a series of insertion operations ：

//  Create a student information sheet 
mysql> create table if not exists Student(
    -> name varchar(20) not null comment ' Student's name , Not allowed to be empty ',
    -> st_id int unsigned unique comment ' Student's student number , Set as unique key ',
    -> telephone varchar(11) primary key comment ' Student's phone number , Set as primary key '
    -> );
Query OK, 0 rows affected (0.03 sec)

//  Observe the structure of the table , Unique key fields are identified UNI
mysql> desc Student;
+-----------+------------------+------+-----+---------+-------+
| Field     | Type             | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| name      | varchar(20)      | NO   |     | NULL    |       |
| st_id     | int(10) unsigned | YES  | UNI | NULL    |       |
| telephone | varchar(11)      | NO   | PRI | NULL    |       |
+-----------+------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

//  test 1： The data of the unique key field cannot be duplicated 
mysql> insert into Student (name, st_id, telephone) values (' Zhang San ', 20204912, '111xxx');
Query OK, 1 row affected (0.00 sec)

mysql> insert into Student (name, st_id, telephone) values (' Li Si ', 20204912, '222xxx');
ERROR 1062 (23000): Duplicate entry '20204912' for key 'st_id'

//  Test two ： The unique key field can be repeatedly empty 
mysql> insert into Student (name, telephone) values (' Zhang San ', '222xxx');
Query OK, 1 row affected (0.01 sec)

mysql> insert into Student (name, telephone) values (' Li Si ', '333xxx');
Query OK, 1 row affected (0.01 sec)

mysql> select * from Student;
+--------+----------+-----------+
| name   | st_id    | telephone |
+--------+----------+-----------+
|  Zhang San    | 20204912 | 111xxx    |
|  Zhang San    |     NULL | 222xxx    |
|  Li Si    |     NULL | 333xxx    |
+--------+----------+-----------+
3 rows in set (0.00 sec)

8. Foreign keys — foreign key

Foreign keys are used to define the relationship between the master table and the slave table ： Foreign key constraints are mainly defined on the slave table , The primary table must have a primary key constraint or unique constraint . When a foreign key is defined , The foreign key field data of the secondary table must exist in the primary key of the primary table （or The only key ） The column exists or is itself NULL.

Understanding foreign keys ：

Foreign key setting method ：
When creating a table, a single row is used to set the foreign key ：

foreign key ( From the field name in the table ) references  Main table ( Field names in the main table )

give an example ：
Create a student table , There are students' names and class numbers in it ; Create another class table , There are subject information corresponding to each class .

Use the foreign key to associate the class number in the student table with the class number in the class table ：

//  The main table must be created first 
mysql> create table if not exists ClaTable(
    -> id int unsigned primary key comment ' Class number ',
    -> course varchar(20) not null comment ' Class courses '
    -> );
Query OK, 0 rows affected (0.02 sec)

//  When creating a slave table, you can select one of your own fields and the fields of the master table for foreign key constraints 
mysql> create table if not exists StuTable(
    -> name varchar(20) not null comment ' The student's name ',
    -> class_id int unsigned comment ' The class the students are in ',
    -> foreign key(class_id) references ClaTable(id)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> desc ClaTable;
+--------+------------------+------+-----+---------+-------+
| Field  | Type             | Null | Key | Default | Extra |
+--------+------------------+------+-----+---------+-------+
| id     | int(10) unsigned | NO   | PRI | NULL    |       |
| course | varchar(20)      | NO   |     | NULL    |       |
+--------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

//  Observed from the table class_id The field key information is MUL
mysql> desc StuTable;
+----------+------------------+------+-----+---------+-------+
| Field    | Type             | Null | Key | Default | Extra |
+----------+------------------+------+-----+---------+-------+
| name     | varchar(20)      | NO   |     | NULL    |       |
| class_id | int(10) unsigned | YES  | MUL | NULL    |       |
+----------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

Let's start with the main table ClaTable Insert two pieces of class related data in ：

mysql> insert into ClaTable (id, course) values (401, ' Physics 、 chemical 、 Geography ');
Query OK, 1 row affected (0.00 sec)

mysql> insert into ClaTable (id, course) values (405, ' history 、 Politics 、 biological ');
Query OK, 1 row affected (0.00 sec)

mysql> select * from ClaTable;
+-----+--------------------------+
| id  | course                   |
+-----+--------------------------+
| 401 |  Physics 、 chemical 、 Geography          |
| 405 |  history 、 Politics 、 biological          |
+-----+--------------------------+
2 rows in set (0.00 sec)

Next, go to the slave table StuTable Insert data into to verify the characteristics of foreign keys ：

• Insert a class number as 500 The students of the class , Because there is no class in the main table , So the insertion failed , At this point, the foreign key constraint is working ：
  
• The data of the foreign key field in the table is allowed to be NULL Of . Insert class from table id by null A student of , Here comes a new student , There is no class assigned yet ：
  

Understand foreign keys and foreign key constraints

The above example , We don't create foreign key constraints , Just set up the student list normally , And the class schedule , We have all the fields we need , Then by comparing the student table and the class table id Information can also associate two tables , here , In actual use , What could be wrong ？

Is it possible that you accidentally input the wrong class number in the inserted student information , But this class is not in the class list , At this time MySQL Not detected , The two tables are business related , But there are no constraints on the business , Then there may be problems in the later business .

The solution is to establish a foreign key constraint relationship between the two tables . The essence of establishing foreign keys is to hand over the relevance to MySQL To review , Let me know in advance MySQL Constraints between tables , So when a user inserts data that does not conform to business logic ,MySQL It will be checked out , You are not allowed to insert .