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use ab;

create table if not exists syl (
id int(4) zerofill primary key auto_increment,     
student_name varchar(20) not null,
cardid varchar(18) not null unique key,
hobby varchar(50));

if not exists	 Indicates whether the table to be created exists , If it doesn't exist, create a table 
int(4) zerofill	 Indicates if the value is less than 4 digit , Then use “0” fill 
auto_increment	 Indicates that this field is a self growing field , That is, each record is automatically incremented 1, The default from the 1 Began to increase ; Self growing field data cannot be duplicated ; Self growing fields must be primary keys ; If the added record data does not specify the value of this field and the addition fails, it will be automatically incremented once 
unique key	 Represents the unique key constraint for this field , The data in this field cannot be repeated ; There can only be one primary key in a table ,  But there can be multiple unique keys in a table 
not null	 Indicates that this field is not allowed to be NULL

Two . clone 、 Copy table

1. Copy all contents of the table （ Including data and table structure ）

create table  The new name of the table  (select * from  Replicated table name )

2. Copy table data or structure

create table  The new name of the table  like  Replicated table name ;  # Copy the format , Can copy the format of the table to the new table , But there's no content 
insert into  The new name of the table  select * from  Replicated table name ;  # Copy the contents of the original table to the new table

3、 ... and . Clear the table 、 Clear the data in the table

1. Method 1

delete from  Table name ;

#delete After emptying the table , There are deleted record entries in the returned result ;delete When working, delete record data line by line ; If there is a self growing field in the table , Use delete from  After deleting all records , The newly added record will be changed from the original largest record  ID  Continue to write the record automatically later

2. Method 2

truncate table  Table name ;

#truncate  After emptying the table , No deleted entries are returned ;truncate  When working, the table structure is rebuilt as it is , So in terms of speed  truncate  than  delete  Empty the watch, quick ; Use  truncate table  After clearing the data in the table ,ID  From  1  Start re recording .

3. Method 3

drop table  Database name . Table name ;

4. Three ways to compare

Delete speed ： drop > truncate > delete
drop To delete a table directly
truncate Refactoring for direct destruction
delete Delete item by item

Security ：
drop and truncate They don't keep records , It is not convenient for data recovery
delete Because there are every step of the delete operation , Therefore, when it is deleted by mistake, the recovery operation can be carried out according to the log
therefore delete The best security

Four 、 Create a temporary table

create temporary table  Table name  ( Field 1  type , Field 2  type  [,primary key ( Primary key name )]);

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
Before exiting the connection , You can also perform operations such as adding, deleting, modifying, and querying , However, you cannot create foreign keys

5、 ... and 、mysql Common constraints

 Primary key constraint   	primary key
 Foreign key constraints 	    foreign key
 Non empty constraint   	not null
 Unique constraint 	    unique [key
 Default constraint 	default
 Self increasing constraint 	   auto_increment

6、 ... and . Foreign key constraints

Definition

If the same attribute field is a primary key in Table 1 , In Table 2, it is not the primary key , This field is called the foreign key of Table 2

effect

Ensure data integrity and consistency

Master and slave tables

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 ）

notes ： The fields of the primary table associated with the foreign key must be set as the primary key , It is required that the slave table cannot be a temporary table , The fields of the master-slave table have the same data type 、 Character length and constraints

After creating the master-slave table , insert data （ First, the main watch , From the watch ）, Delete data records （ First from the watch , Back main table ）, Delete foreign key constraint fields （ Delete the foreign key constraint first , Then delete the foreign key name ）

7、 ... and . User management

1. A new user

create user ' user name '@' Source address ' [identified by [passwd] ' password '];

user name ： Specify the user name that will be created
Source address ： Specify which hosts the newly created user can log on to , You can use IP Address 、 Network segment 、 The form of host name , Available to local users localhost, Allow any host to log in
Wildcards available %

password ’： If plaintext password is used , Direct input ’ password ’, Inserted into the database by Mysql Automatic encryption ;
If you use an encrypted password , You need to use it first SELECT PASSWORD(‘ password ’); Get ciphertext , Then add... To the statement PASSWORD ‘ Ciphertext ’;
If you omit “IDENTIFIED BY” part , Then the user's password will be empty （ Not recommended ）

2. View user information

use mysql;

select User,authentication_string,Host from user;

3. Rename user

rename user 'lisi'@'localhost' to 'wangwu'@'localhost';

4. Delete user

drop user ' user name '@'localhost';

5. Change the password of the current login user

set password = password(' New password ');

6. Change other user passwords

set password for ' user name '@'localhost' = password(' New password ');
use mysql;
select User,authentication_string,Host from user;

8、 ... and . User authorization

1. Give authority

grant  Raise the right 
grant  Permission list  on  Database name . Table name  to ' user name '@' Source address ' [identified by ' password '];

Permission list ： Used to list various database operations authorized for use , Separated by commas （all Express all ）
Source address ： Used to specify the user name and the client address to which access is allowed , The source address can be a domain name 、IP Address , You can also use “%” wildcard

2、 View permissions

show grants for  user name @ Source address ;

3、 Revoke authority

revoke  Permission list  on  Database name . Table name  from  user name @ Source address ;

summary

mysql To create a new user, you need to specify permissions

There are two ways to copy tables ：1 Copy the table contents and structure together 2. The table contents and structure are copied separately

The function of foreign keys is to ensure the integrity and consistency of data

The temporary table can be used normally before the user exits , The table will be automatically destroyed after exiting

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