I. creation and constraint of MySQL table

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Preface

The last blog shared MySQL Download and configuration of environment variables , And simply create the database and tables . This blog will share the syntax of database creation and destruction and explain it in detail MySQL The creation code of tables and the use of constraints , Strive for one article to grasp these knowledge points ！

One 、 Establishment and destruction of database

Building a database ：

create database [if not exists]  Database name  [default charset  Character set ]

tips： brackets [] The contents can be omitted

• add if not exists The meaning is ： If this database does not exist, it will be created
• default charset Character set Is to set the character set when creating the database

Destroy database ：

drop database [if exists]  Database name 

tips： The meanings of the parts that can be omitted are the same as above

Two 、 Creation and destruction of tables

1、 The establishment of tables

grammar ：

create table [if not exists]  Table name (
	 Field name    data type   [ constraint ],
     Field name    data type   [ constraint ],
    ...( The last field cannot have a comma )
)

1.1、 Naming rules

1. Table names are usually in the form of t_ start
2. Key fields are usually represented by Table name abbreviation _ start
3. MySQL Case insensitive , Cannot use keyword naming , Multiple parts are underlined

1.2、 data type

Data types to master ：

1. Integers ：int、integer、bigint
2. decimal ：decimal
3. character string ：varchar
4. enumeration ：enum
5. date ：datatime

Use advice ：

1. When the field number must be greater than 0 And unsigned numbers can be used without subtraction unsigned, It is usually not recommended to
2. float and double There is Precision problem , follow-up mysql Will no longer support
3. char and verchar The underlying implementation is consistent when saving multiple characters , It's just that the mode of occupying space is different , Not recommended char, Because it may waste space
4. timestamp The upper limit is too low ,date and time Unpractical , Therefore, it is suggested to use datetime

2、 Table constraint concept

2.1、 Constraint Preview

constraint	explain	explain
primary key	Primary key	Used to mark a column as the primary key column
not null	Non empty	Constraint the column value cannot be empty
unique	only	Constrain that the column value cannot be repeated
default	Default	Set the default value for this column
foreign key	Foreign keys	The flag column is the foreign key value
check	Check constraint	MySQL I won't support it

2.2、 Characteristics and functions

• Primary key
  • effect ： It can be used as a unique identifier to specifically lock a row of data
  • characteristic ： Non empty and unique
    • A table can have at most one primary key column
• Non empty
  • characteristic ： The value can not be null, Must set
• only
  • characteristic ： The value can be null, And there can be multiple null, This constraint is only valid for values that already exist
    Non empty constraint + Unique constraint Not equal to Primary key constraint
• Default
  • characteristic ： If the column is not explicitly assigned , Then use the default value to store
  • grammar ： default( value )

The use of foreign keys is explained in detail below

3、 Table constraints are specifically used

3.1、 Column level constraints

Define the constraint directly in the declaration of the column
Examples of use ：

-- id、 full name 、 Age 、 Gender 、 height 、 Birthday 
create table if not exists t_person(
		p_id int primary key,--  Primary key 
		p_name varchar(50) unique not null,--  only + Non empty 
		age tinyint unsigned not null,--  Non empty   An unsigned number  0-255
		sex enum(' male ',' Woman ',' Ultraman ') default ' Ultraman ',--  Default 
		height decimal(4,1), --  All in all 4 position , The decimal point takes up 1 position ：000.0
		birthday datetime
)

3.2、 Table level constraints

Define constraints separately below all columns ,not null and default No table level constraints
grammar ：[constraint Custom constraint name ] Constraint type （ Field name ）
Examples of use ：

create table if not exists t_person(
		p_id int,--  Primary key 
		p_name varchar(50) not null,--  only + Non empty 
		age tinyint unsigned not null,--  Non empty   An unsigned number  0-255
		sex enum(' male ',' Woman ',' Ultraman ') default ' Ultraman ',--  Default 
		height decimal(4,1), --  All in all 4 position , The decimal point takes up 1 position ：000.0
		birthday datetime,
		--  Table level constraints 
		primary key(p_id),--  to p_id Add primary key constraint 
		constraint un_name unique(p_name)--  to p_name Add unique constraints 
)

3.3、 Foreign key constraints

• effect ： Used to indicate the relationship between two tables , Must be used in conjunction with another table
• characteristic ：
  1. The connection must be from another table Primary key or unique column
  2. The value of the foreign key column You can't customize , Must be from a value that already exists in the column associated with another table
  3. The value can be null, Is not the only
  4. Must use Table level constraints establish
• grammar ：
  foreign key( Field name ) references Other table names ( Connection field names of other tables )
• Use ：
  1. Table creation sequence ： First create a table without foreign keys , Create a table with foreign keys
  2. Deletion order ： Delete the foreign key first , Delete any foreign keys
  3. Foreign key column data type Must be consistent with the connected column

Examples of use ：

--  Class table ：id, Class name 
create table t_class(
	c_id  int PRIMARY KEY,
	c_name varchar(20) not null
)
--  Student list ：id, full name , Class id
create table t_student(
	s_id int PRIMARY KEY,
	s_name varchar(20) not null,
	c_id int not null ,
	--  Table level constraints 
	FOREIGN KEY(c_id) REFERENCES t_class(c_id)
)

4、 Destruction of tables

grammar ：
drop table [if exists] Table name
matters needing attention ：
stay 3.3 Foreign key constraints are specifically used for tables with foreign keys , Delete the table with foreign keys first , Delete the table without foreign keys
drop table t_student – Delete the student list first
drop table t_class – To delete the class table

Conclusion

I hope this blog can help you , Let's make progress together ！！！