Basic MySQL operation commands of database

One 、 database structure

database –> Data sheet –> That's ok （ Record ）： Information used to describe an object
Column （ Field ）： Used to describe an attribute of an object

Two 、 Common data types

Common data types	effect
int	integer
float	Single precision floating point -------4 byte 32 position
double	Double precision floating point -------8 byte 64 position
char	Fixed length character types
varchar	Variable length character types
text	Text
image	picture
decimal (5,2)	5 A valid length number , After the decimal point is 2 position

Add ：
char The maximum number of characters can be stored 255 individual ,char If the actual length of the stored data is smaller than the specified length , The space will be filled to the specified length ; If the actual length of the stored data is greater than the specified length , Lower versions will be intercepted , The higher version will report an error .
char The length of is immutable , and varchar The length of is variable , in other words , Define a char[10] and varchar[10], If the deposit is ‘csdn’, that char The total length is still 10, Except for the characters ‘csdn’ Outside , Followed by six spaces , and varchar Immediately change the length to 4 了

varchar Storage rules ：
4.0 Version below ,varchar(20), refer to 20 byte , If stored UTF8 Chinese character time , Can only save 6 individual （ Every Chinese character 3 byte ）
5.0 Above version ,varchar(20), refer to 20 character , It doesn't matter what the numbers are 、 The letters are still UTF8 Chinese characters （ Every Chinese character 3 byte ）, All can be stored 20 individual , The biggest size is 65532 byte .

3、 ... and 、 Check the database structure

• Enter the database before using the command
  

1、 View the database in the current server

2、 Look at the tables contained in the database

3、 View the structure of the table

Four 、SQL sentence

SQL Statement is used to maintain the management database , Including data query 、 Data update 、 Access control 、 Object management and other functions .

SQL Language	function
DDL	Data definition language , Used to create database objects , Ruku 、 surface 、 Index, etc.
DML	Data manipulation language , It is used to manage the data in the table
DQL	Data query language , Used to find eligible data records from the data table
DCL	Data control language , Used to set or change database user or role permissions

1、DDL（ Data definition language ）

（1） create new database

（2） Create a new table

（3） Delete the specified data table

（4） Delete the specified database

2、DML（ Data operation language ）

（1） Manage data records in tables

Insert a new data record into the data table

Query data records

modify 、 Update data records in data tables


Delete the specified data record in the data table

（2） Modify the table name and structure

Modify the name of the table

Extended table structure （ Add fields ）

Modify fields （ Column ） name , Add unique key

Delete field

3、 Expand

5、 ... and 、 Data table advanced operations

1、 Clone table , Generate the data records of the data table into a new table

1、 Method 1 ： Use like

2、 Method 2 ： Direct cloning

• The defect of direct cloning ： Some special attributes cannot be cloned
  

2、 Clear the table , Delete all data in the table

Method 1 ： Record ID Not delete

Method 2 ： Delete record ID

3、 Create a temporary table

After the temporary table is created successfully , Use SHOW TABLES The command cannot see the created temporary table , The temporary table will be destroyed after the connection exits . If before exiting the connection , You can also perform operations such as adding, deleting, modifying, and querying , For example, use DROP TABLE Statement to manually and directly delete the temporary table .

4、 Create foreign key constraints , Ensure data integrity and consistency

• Definition of foreign key ： If the same attribute field X In Table 1 is the primary key , In Table 2, it is not the primary key , Then the fields X The foreign key called table 2
• Understanding of primary key table and foreign key table ：
  • Tables with public keywords as primary keys are primary key tables （ Parent table 、 Main table ）
  • Tables with public keywords as foreign keys are foreign key tables （ From the table 、 appearance ）
    Be careful ： The fields of the child table associated with the foreign key must be set as the primary key . The foreign key fields of the main table and the fields of the sub table are required to have the same data type 、 Character length and constraints .
    

• When inserting a new data record , The master table first and then the slave table
  
• When deleting data records , First from the main table and then from the main table , In other words, when deleting a primary key table, you must first delete other tables associated with it
  

6、 ... and 、 Database user management

1、 A new user

2、 View user information

3、 Rename user

4、 Delete user

5、 Modify the password of the current login user

7、 ... and 、 Database user authorization

1、 Grant authority

• GRANT sentence ： It is specially used to set the access rights of database users .
  • When the specified user name does not exist ,GRANT Statement will create a new user ;
  • When the specified user name exists ,GRANT Statement is used to modify user information .
    

2、 Revoke authority