SQL statement

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 classification :
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

Application SQL Statement instance

mysql> create table if not exists jjz (     
    -> id int(4) zerofill primary key auto_increment,
    -> name varchar(10) not null,
    -> cardid int(18) not null unique key,
    -> hobby varchar(50));
    
    #if not exists: Indicates whether the table to be created exists , If it doesn't exist, continue to create 
    #int(4) zerofill: Indicates if the value is less than 4 digit , Then use “0” fill 
    #auto_increment: Indicates that the secondary field is a self increasing 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 a secondary field and the addition fails, it will also be automatically incremented once 
    #unique key: Represents the secondary field unique key constraint , 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

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Data table advanced operations

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

The first method

mysql> create table  New table  like  Old table ;

 notes ： Just clone the structure of the table 

Clone old table data to new table

mysql> insert into  New table  select * from  Old table ;

The second method

Directly clone table structure and table data

mysql> create table  New table  (select * from  Old table );

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Clear the table , Delete all data in the table

Method 1

mysql> delete from  Table name ;
#delete After emptying the table , There are deleted entries in the returned results ;delete Delete the recorded data line by line when working ; If there is a self growing field in the table , Use delete from  After deleting all records , The newly added record will change from the original maximum record ID Continue to write the record automatically later .

Method 2

mysql> truncate table  Table name ;
#truncate  After emptying the table , There are deleted record entries in the returned result ;truncate When working, the table structure is rebuilt as it is , So in terms of speed ,truncate  than  delete Empty the watch, quick ; After using the data in the table ,ID From 1  Start re recording 

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Create a temporary table

Look at the data in the table

But when looking at the tables in the database when Temporary tables are invisible , And only the current connection is valid , After logging out of the connection, it will be automatically destroyed

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Create foreign key constraints , Ensure the integrity and unity of data

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 :
(1) Tables with public keywords as primary keys are primary key tables ( Parent table 、 Main table )

(2) Tables with public keywords as foreign keys are foreign key tables （ From the table 、 appearance )
Be careful : 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 .

First, create two tables

Two points need to be noted here , When inserting data , First add from the main table and then add in the foreign key table , First the main table and then the appearance

When deleting data , Delete the appearance record first and then the main table record

View foreign key constraints and delete foreign key constraints

MySQL in 6 A common constraint ：

Primary key constraint (primary key)

Foreign key constraints ( foreign key )

Non empty constraint (not null)

Uniqueness constraint (unique [key l index] )

Default constraint ( default)

Self increasing constraint (auto_ increment)

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Mysql DCL sentence

Create user password encryption

Database account user information storage location

mysql Remote login

rename user

Delete user name

Change the current user password

Forget the password , How to deal with it

Exit database

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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 .

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 , Such as  "select, insert, updatel". Use "al1" Indicates all permissions , You can authorize any operation .
        
    # Database name . Table name : The name of the database and table used to specify the authorization operation , You can use wildcards "*". for example , Use "kac." The object representing the authorization operation is kac All the tables in the database .
        
    ' user name @ Source address ': Used to specify the user name and the client address to which access is allowed , Who can connect 、 Where can I connect . The source address can be a domain name 、IP Address , You can also use "%" wildcard , Represents all addresses in an area or network segment , Such as "%. kgc.com”、"192.168.80." etc. .
        
    #IDENTIFIED BY: Used to set the password string used by users when connecting to the database . When creating a new user , If you omit "*IDENTFIED BY" part , Then the user's password will be empty .

Set the local login permission of the account

Open another port

Create an account And set the permission to allow login on all hosts , Have permission to view all tables

Switch to another host to verify login

View the permission of an account

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How to revoke the authority of an account