SQL operation database syntax

SQL Concept understanding and usage of

What is? SQL:

Structured Query Language:  Structured query language ,
 In fact, it defines the rules for operating all relational databases .
 Relational database :
	Reational DBMS;

SQL The general syntax of :

· SQL Statements can be written in one or more lines , It ends with a semicolon .

· You can use spaces and indents to enhance the readability of the language .

· MySql Database SQL Statement is case insensitive . It is recommended to use uppercase .
SELECT * FROM student

· Three notes W
Single-line comments : – The comment or # The comment (Mysql specific ) Multiline comment /* notes */

SQL The classification of

SQL The classification of :
1): DDL(Data Definition Language) Data definition language
Used to define database objects : database , surface , Column , etc. . keyword : caeate, drop, alter etc.

2): DML(Data Manipulation Language) Data operation language
It is used to add, delete and modify data in the database . keyword : insert, delete, update etc.

3): DQL(Data Query Language) Data query language
Used to query the records of tables in the database ( data ). keyword : select, where etc. .

4): DCL(Data Control Language) Data control language ( understand )
Used to define database access and security levels , And create users . keyword : GRANT, REVOKE etc. .

The first point DDL: Operating the database _ Create a query

DDL:  Operating the database 、 surface :
	
	1.  Operating the database : CRUD
		C(Create): establish 		R(Retrieve):  Inquire about 	  U(Update):  modify 	   D(Delete):  Delete 		 Using a database 
		
	C:	
*  Create database :
	* create database  Database name ;

	*  Determine whether the database exists . If it doesn't exist, I'm creating , Existence contrary .
		* create database if not exists  Database name ;

	*  Change the character set when creating the database ( Default utf-8)
		* create database  Database name  character set  Character set ;

	*  subject :  establish db4  database , Judge whether it exists , And set the character set to gbk;
		* create database if not exists db4 character set gbk;

	R:
*  Find the names of all databases :
	* show databases;
*  View the character set of the corresponding database : *( View the creation statement of a database )
	* show create database mysql;	SHOW CREATE DATABASE mysql;		(mysql  Is the corresponding database , You can change to the database you want to view .)

	U:
*  Modify the character set of the database :
	* alter database  Database name  character set  Character set name ;

	D:
*  Delete database :
	* drop database  Database name ;

	*  Judge its existence   Delete if it exists . On the contrary, inaction :
		* drop database if exists  Database name ;

	 Using a database :
*  Using a database 
	* use  Database name ;

*  Query the name of the database currently in use 
	* select database();

---

DDL Operation table _ Inquire about _ establish _ modify _ Delete

C(Create): establish 
	* create table  Table name (
		 Name 1  data type 1, 
		 Name 2  data type 2,
		...
		 Name n  data type n
		);

	*** SQL Common data types :
			int   Integer types ;

			double  Decimal type ;
				double(5 ,2);	//  The maximum number of decimals is five , Keep two decimal places .

			date   The date type , The date type only includes year, month and day . yyyy-MM-dd
			datetime   The date type ,  contain   Mm / DD / yyyy HHM / S  yyyy-MM-dd HH:mm:ss;
			timestamp   Timestamp type ;  contain   Mm / DD / yyyy HHM / S  yyyy-MM-dd HH:mm:ss;
				*  If you don't assign a value to this field in the future , Or assign it to null; The current system time is used by default , To automatically assign .

			varchar;  Character type 
				* name varchar(20); //  The maximum character length is 20 position ;



R(Retrieve):  Inquire about 	  
	*  Query all table names in a database :
		* show tables;

	*  Query table structure :
		* desc  Table name ;

U(Update):  modify 	   
	*  Modify the name of the table  : alter table  Table name  rename to  The new name of the table ;

	*  View the character set of the table : show create table  Table name ;

	*  Modify the character set of the table : alter table  Table name  character set  Character set ;

	*  Add a column : alter table  Table name  add  Name   data type ;

	*  Modify the name and type of the column : alter table  Table name  change  Old column names    New column names   data type ;
		*  Just change the type : alter table  Table name  modify  Name   data type ;

	*  Delete column  : alter table  Table name  drop  Name ;

D(Delete):  Delete 
	*  Delete table :
		* drop table  Table name ;
		*  Judge whether it exists in delete ; On the contrary, inaction :
			* drop tabele if exists  Table name ;


 Copy table :  create table  New table  like  The copied table ;

---

SQL Common methods ( Some are continuously updated ):

 It is recommended that all statements be capitalized ！

*** SQL Common data types :
	int   Integer types ;

	double  Decimal type ;
		double(5 ,2);	//  The maximum number of decimals is five , Keep two decimal places .

	date   The date type , The date type only includes year, month and day . yyyy-MM-dd
	datetime   The date type ,  contain   Mm / DD / yyyy HHM / S  yyyy-MM-dd HH:mm:ss;
	timestamp   Timestamp type ;  contain   Mm / DD / yyyy HHM / S  yyyy-MM-dd HH:mm:ss;
				*  If you don't assign a value to this field in the future , Or assign it to null; The current system time is used by default , To automatically assign .

	varchar;  Character type 
		* name varchar(20); //  The maximum character length is 20 position ;

 Create database :
	create database  Database name ;

	*  Judge whether there is . If it doesn't exist, I'm creating , Existence contrary .
	    * create database if not exists  Database name ;

	*  Change the character set when creating the database ( Default utf-8)
		* create database  Database name  character set  Character set ;


 Delete database :
	drop database  Database name ;

	*  Judge its existence   Delete if it exists . On the contrary, inaction :
		* drop database if exists  Database name ;


 Using a database :( It can also be said to enter the database )
	* use  Database name ;

	*  Query the name of the database currently in use 
		* select database();


 Query all databases :
	show databases;		SHOW DATABASES;


 View the character set of the corresponding database : *( View the creation statement of a database )
	show create database mysql;	SHOW CREATE DATABASE mysql;		(mysql  Is the corresponding database , You can change to the database you want to view .)


 Modify the character set of the database :
	alter database  Database name  character set  Character set name ;



 Operation of the watch ;

 Copy table :  create table  New table  like  The copied table ;

C(Create): establish 
	* create table  Table name (
		 Name 1  data type 1, 
		 Name 2  data type 2,
		...
		 Name n  data type n
		);


R(Retrieve):  Inquire about 	  
	*  Query all table names in a database :
		* show tables;

	*  Query table structure :
		* desc  Table name ;


D(Delete):  Delete 
	*  Delete table :
		* drop table  Table name ;
		*  Judge whether it exists in delete ; On the contrary, inaction :
			* drop tabele if exists  Table name ;


U(Update):  modify 	   
	*  Modify the name of the table  : alter table  Table name  rename to  The new name of the table ;

	*  View the character set of the table : show create table  Table name ;

	*  Modify the character set of the table : alter table  Table name  character set  Character set ;

	*  Add a column : alter table  Table name  add  Name   data type ;

	*  Modify the name and type of the column : alter table  Table name  change  Old column names    New column names   data type ;
		*  Just change the type : alter table  Table name  modify  Name   data type ;

	*  Delete column  : alter table  Table name  drop  Name ;



DML:  Add or delete data in the table