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How to distinguish and define DQL, DML, DDL and DCL in SQL

2022-06-28 10:00:00 【51CTO】

SQL(Structured Query Language) Structured query language is the core language of database , It's a high-level nonprocedural programming language , It is also the implementation of the classic declarative programming paradigm .
SQL General grammar

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 statements .
MySQL Database SQL Statement is case insensitive , It is recommended to use uppercase .
notes
- Single-line comments : – The comment or # The comment (mysql specific )
- Multiline comment : / notes /

DDL: Operating the database 、 surface

DDL（Data Definition Language） Data definition language , It mainly operates databases and tables , Database related operations include database creation , Delete Library , Table related operation creation table 、 Delete table 、 Modify table fields 、 Modify constraints and other operations , Keywords generally include ：create、drop、alter、truncate、comment,rename.

Operating the database ：CRUD

C(Create): establish

--  Create database ：
create database  Database name ;

--  Create database , Judgment does not exist , To create a ：
create database if not exists  Database name ;

--  Create database , And specify the character set 
create database  Database name  character set  Character set name ;

--  practice ：  establish db4 database , Judge whether it exists , And make character set as gbk
create database if not exists db4 character set gbk;

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R(Retrieve)： Inquire about

--  Query all database names :
show databases;
--  Query the character set of a database : Query the creation statement of a database 
show create database  Database name ;

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U(Update): modify

D(Delete): Delete

--  Delete database 
drop database  Database name ;

--  Determine that the database exists , Exist and delete 
drop database if exists  Database name ;

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Using a database

Operation table

C(Create): establish

grammar ：

create table Table name (
Name 1 data type 1,
Name 2 data type 2,
…
Name n data type n
);
Be careful ： The last column , There is no need to add comma ,

Database type ：

int： Integer types
age int,
double: Decimal type
score double(5,2)
date: date , Include only mm / DD / yyyy ,yyyy-MM-dd
datetime: date , Including month, day, hour, minute, second yyyy-MM-dd HH:mm:ss
timestamp: Wrong type of time Including month, day, hour, minute, second yyyy-MM-dd HH:mm:ss
If you don't assign a value to this field in the future , Or assign the value to null, The current system time is used by default , To automatically assign
varchar： character string
name varchar(20): The biggest name 20 Characters
zhangsan 8 Characters Zhang San 2 Characters

Create a table case ：
create table student(
id int,
name varchar(32),
age int ,
score double(4,1),
birthday date,
insert_time timestamp
);
Copy table ：

create table Table name like The name of the copied table ;

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  New table name ;

--  Modify the character set of the table 
alter table  Table name  character set  Character set name ;

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

--  Change column name   type 
alter table  Table name  change  Name   New listing   New data types ;
alter table  Table name  modify  Name   New data types ;

--  Delete column 
alter table  Table name  drop  Name ;

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D(Delete): Delete

DML： Add, delete and modify the data in the table

DML（Data Manipulation Language） Data operation language , Do some simple operations on the data in the database , There are generally three keywords ：insert, delete, update.

--  Add data ：
--  grammar ：
insert into  Table name ( Name 1, Name 2,... Name n) values( value 1, value 2,... value n);
--  Be careful ：
-- 1.  Column name and value should correspond one by one .
-- 2.  If the name comes after , No column name defined , The default is to add values to all columns 
insert into  Table name  values( value 1, value 2,... value n);
-- 3.  Except for the number type , Other types require quotation marks ( Single and double can ) Lead up 

--  Delete data ：
--  grammar ：
delete from  Table name  [where  Conditions ]
--  Be careful ：
-- 1.  If there is no condition , Then delete all records in the table .
-- 2.  If you want to delete all records 
delete from  Table name ; --  It is not recommended to use . How many records will be deleted 
TRUNCATE TABLE  Table name ; --  Recommended , More efficient   Delete the table first , Then create the same table .

--  Modifying data ：
--  grammar ：
update  Table name  set  Name 1 =  value 1,  Name 2 =  value 2,... [where  Conditions ];
--  Be careful ：
-- 1.  Without any conditions , All records in the table will be modified .

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DQL： Look up the records in the table

DQL(Data Query Language） Data query language , Data retrieval statements , Used to get data from a table . Usually the most commonly used is reserved words select. The commonly used statement keywords are ：SELECT, FROM, WHERE, ORDER BY, HAVING,ASC|DESC etc. .

select  from  Table name ;

--  grammar ：
select
	 Field list 
from
	 List of table names 
where
	 List of conditions 
group by
	 Grouping field 
having
	 Conditions after grouping 
order by
	 Sort 
limit
	 Paging limit 
	

--  Basic query 
-- 1.  Multiple field queries 
select  Field name 1, Field name 2... from  Table name ;
--  Be careful ： If you query all fields , You can use to replace the field list .
-- 2.  Remove duplication ：
distinct
-- 3.  Calculated column 
 Generally, four operations can be used to calculate the values of some columns .（ Generally, only numerical calculation will be carried out ）
ifnull( expression 1, expression 2)：null Operations involved , The results are null
	 expression 1： Which field needs to determine whether it is null
	 If the field is null The replacement value after .
-- 4.  names ：
as：as You can omit it 
	

--  Conditions of the query 
-- 1. where Clause followed by condition 
-- 2.  Operator 
	 > 、< 、<= 、>= 、= 、<>
	 BETWEEN...AND  
	 IN(  aggregate ) 
	 LIKE： Fuzzy query 
		  Place holder ：
			 _: Any single character 
			 %： More than one arbitrary character 
	 IS NULL  
	 and   or  &&
	 or   or  || 
	 not   or  !

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DCL： Manage users , to grant authorization

DCL（Data Control Language） Data control language , Such as grant to grant authorization 、revoke Revocation of authority, etc , It is mainly used to control access rights or command submission .

Manage users

Add users ：

grammar ：CREATE USER ‘ user name ’@‘ Host name ’ IDENTIFIED BY ‘ password ’;

Delete user ：

grammar ：DROP USER ‘ user name ’@‘ Host name ’;

Change user password ：

UPDATE USER SET PASSWORD = PASSWORD(' New password ') WHERE USER = ' user name ';
UPDATE USER SET PASSWORD = PASSWORD('abc') WHERE USER = 'lisi';
SET PASSWORD FOR ' user name '@' Host name ' = PASSWORD(' New password ');
SET PASSWORD FOR 'root'@'localhost' = PASSWORD('123'); 

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mysql I forgot root User's password ？
1. cmd – > net stop mysql stop it mysql service
- The administrator is required to run the cmd
2. Start with no validation mysql service ： mysqld --skip-grant-tables
2. Open up new cmd window , Direct input mysql command , Knock back . You can log in successfully
2. use mysql;
2. update user set password = password(‘ Your new password ’) where user = ‘root’;
2. Close two windows
2. Open Task Manager , Manual end mysqld.exe The process of
2. start-up mysql service
2. Log in with the new password .

Query the user ：

-- 1.  Switch to mysql database 
USE mysql;
-- 2.  Inquire about user surface 
SELECT * FROM USER; 
--  wildcard ： %  Indicates that the user can log in to the database on any host 

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Rights management

Query authority ：

Grant authority

--  Grant authority 
grant  Permission list  on  Database name . Table name  to ' user name '@' Host name ';

--  Give all permissions to Zhang San , On any table in any database 
GRANT ALL ON *.* TO 'zhangsan'@'localhost';

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Revoke authority

--  Revoke authority ：
revoke  Permission list  on  Database name . Table name  from ' user name '@' Host name ';
REVOKE UPDATE ON db3.`account` FROM 'lisi'@'%';

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TCL： Transaction control

TCL（Transaction Control Language） Transaction control language ： Such as commit Commit transaction ,rollback Roll back the transaction set transaction Modify the transaction isolation level of the database or the access mode of the table data in the transaction .

Concept ：

If a business operation contains multiple steps , Managed by affairs , So these operations are either successful at the same time , Or fail at the same time .

operation ：

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SQL Medium DQL、DML、DDL and DCL How to distinguish and define _ Table name