MySQL Basics (concepts, common instructions)

database

Concept

1. database ：
   An area of a disk dedicated to storing data
   

• To operate the data in the database, we need to use structured query language （Structured Query Language）, namely SQL To achieve
• SQL Is the language that interacts with the database , Be careful ,SQL Not for one DBMS, Almost all DBMS All support SQL, But for different DBMS Come on , They achieve sQL It's not exactly the same

2. Database management system
   A specific software or system that manages databases , Also known as DBMS（DataBase Management System）
   
   Our operation of database , All based on DBMS, Instead of directly manipulating the database

3. database server
   A physical computer , Database installed on it , Specifically used to access data , We can use it remotely DBMS Operate the database

Commonly used instructions

• DDL: Data definition language , be used for Define database objects （ database 、 surface 、 Field ）
• DML: Data operation language , It is used to Additions and deletions
• DQL: Data query language , be used for check Query the records of tables in the database
• DCL: Data control language , be used for Create database users 、 Control the control permissions of the database

DDL( Data definition language )

1. Database level

• Query all databases ：
  SHOW DATABASES;
• Querying the current database ：
  SELECT DATABASE();
• Create database ：
  CREATE DATABASE [ IF NOT EXISTS ] Database name [ DEFAULT CHARSET Character set ] [COLLATE Sort rule ];
• Delete database ：
  DROP DATABASE [ IF EXISTS ] Database name ;
• Using a database ：
  USE Database name ;
  notes ：UTF8 The character set length is 3 byte , Some symbols account for 4 byte , Recommend to use utf8mb4 Character set

2. Surface level

• Query all tables in the current database ：
  SHOW TABLES;

• Query table structure ：
  DESC Table name ;

• Query the table creation statement of the specified table ：
  mysql SHOW CREATE TABLE Table name ;

• Create table ：
  mysql CREATE TABLE Table name ( Field 1 Field 1 type [COMMENT Field 1 notes ], Field 2 Field 2 type [COMMENT Field 2 notes ], Field 3 Field 3 type [COMMENT Field 3 notes ], ... Field n Field n type [COMMENT Field n notes ] )[ COMMENT Table annotation ];

• Add fields ：
  ALTER TABLE Table name ADD Field name type ( length ) [COMMENT notes ] [ constraint ];

• Change data type ：
  ALTER TABLE Table name MODIFY Field name New data types ( length );

• Modify the field name and field type ：
  ALTER TABLE Table name CHANGE Old field name new field name type ( length ) [COMMENT notes ] [ constraint ];

• Delete field ：
  ALTER TABLE Table name DROP Field name ;

• Modify the name of the table ：
  ALTER TABLE Table name RENAME TO The new name of the table

• Delete table ：
  DROP TABLE [IF EXISTS] Table name ;

• Delete table , And recreate the table ：
  TRUNCATE TABLE Table name ;

DML（ Data operation language ）

1. Add data

• Specified field ：
  INSERT INTO Table name ( Field name 1, Field name 2, ...) VALUES ( value 1, value 2, ...);

• All fields ：
  INSERT INTO Table name VALUES ( value 1, value 2, ...);

• Batch add data ：
  INSERT INTO Table name ( Field name 1, Field name 2, ...) VALUES ( value 1, value 2, ...), ( value 1, value 2, ...), ( value 1, value 2, ...);
INSERT INTO Table name VALUES ( value 1, value 2, ...), ( value 1, value 2, ...), ( value 1, value 2, ...);
  notes ： String and date type data are enclosed in quotation marks

2. Delete data

• Delete data ：
  DELETE FROM Table name [ WHERE Conditions ];

3. Change data

• Modifying data ：
  UPDATE Table name SET Field name 1 = value 1, Field name 2 = value 2, ... [ WHERE Conditions ];

DQL（ Data query language ）

grammar ：

SELECT
	 Field list 
FROM
	 Table name field 
WHERE
	 List of conditions 
GROUP BY
	 Group field list 
HAVING
	 List of conditions after grouping 
ORDER BY
	 Sort field list 
LIMIT
	 Paging parameters 

1. Basic query

• Query multiple fields ：
  SELECT Field 1, Field 2, Field 3, ... FROM Table name ;

• Query all fields ：
  SELECT * FROM Table name ;

• Set alias ： Use as keyword ,as Omission
  SELECT Field 1 [ AS Alias 1 ], Field 2 [ AS Alias 2 ], Field 3 [ AS Alias 3 ], ... FROM Table name ;
SELECT Field 1 [ Alias 1 ], Field 2 [ Alias 2 ], Field 3 [ Alias 3 ], ... FROM Table name ;

• Remove duplicate records ：
  SELECT DISTINCT Field list FROM Table name ;

2. Conditions of the query

grammar ：

SELECT  Field list  FROM  Table name  WHERE  List of conditions ;

Conditions ：

Comparison operator	function
>	Greater than
>=	Greater than or equal to
<	Less than
<=	Less than or equal to
=	be equal to
<> or !=	It's not equal to
BETWEEN … AND …	In a certain range （ Containing minimum 、 Maximum ）
IN(…)	stay in The values in the following list , A commonplace
LIKE Place holder	Fuzzy matching （_ Match a single character ,% Match any character ）
IS NULL	yes NULL

Logical operators	function
AND or &&	also （ Multiple conditions hold at the same time ）
OR or ||	perhaps （ Any one of several conditions holds ）
NOT or !	Not , No

give an example ：

--  Age equals 50
select * from employee where age = 50;
--  Age is less than 50
select * from employee where age < 50;
--  Less than or equal to 50
select * from employee where age <= 50;

--  No ID card 
select * from employee where idcard is null or idcard = '';
--  Have ID card 
select * from employee where idcard;
select * from employee where idcard is not null;

--  It's not equal to 
select * from employee where age != 50;
--  Age 30 To 50 Between 
select * from employee where age between 30 and 50;
select * from employee where age >= 30 and age <= 50;

--  Don't complain , But I can't find any information 
select * from employee where age between 50 and 30;
--  The gender is female and the age is less than 30
select * from employee where age < 30 and gender = ' Woman ';
--  Age equals 25 or 30 or 35
select * from employee where age = 25 or age = 30 or age = 35;
select * from employee where age in (25, 30, 35);

--  The name is two words 
select * from employee where name like '__';
--  The last ID card is X
select * from employee where idcard like '%X';

3. Aggregate query （ Aggregate functions ）

Common aggregate functions ：

function	function
count	Statistical quantity
max	Maximum
min	minimum value
avg	Average
sum	Sum up

grammar ：

SELECT  Aggregate functions ( Field list ) FROM  Table name ;

example ：
SELECT count(id) from employee where workaddress = " Guangdong province, ";

4. Group query

grammar ：

SELECT  Field list  FROM  Table name  [ WHERE  Conditions  ] GROUP BY  Group field name  [ HAVING  Filter conditions after grouping  ];

where and having difference ：

• The timing of execution is different ：where Is to filter before grouping , dissatisfaction where Condition does not participate in grouping ;
  having Yes, filter the results after grouping .
• Different judgment conditions ：where Aggregate functions cannot be judged , and having Sure .

Example ：

--  Group by sex , Count the number of men and women （ Show only the number of groups , It doesn't show which is male and which is female ）
select count(*) from employee group by gender;
--  Group by sex , Count the number of men and women 
select gender, count(*) from employee group by gender;
--  Group by sex , Count the average age of men and women 
select gender, avg(age) from employee group by gender;
--  Age is less than 45, And grouped according to the working address 
select workaddress, count(*) from employee where age < 45 group by workaddress;
--  Age is less than 45, And grouped according to the working address , Get the number of employees greater than or equal to 3 My work address 
select workaddress, count(*) address_count from employee where age < 45 group by workaddress having address_count >= 3;

notes :

• Execution order ：where > Aggregate functions > having
• After grouping , The query fields are generally aggregate functions and grouping fields , Querying other fields makes no sense

5. Sort query

grammar ：
SELECT Field list FROM Table name ORDER BY Field 1 sort order 1, Field 2 sort order 2;

sort order ：

• ASC: Ascending （ Default ）
• DESC: Descending

Example ：

--  Sort in ascending order by age 
SELECT * FROM employee ORDER BY age ASC;
SELECT * FROM employee ORDER BY age;
--  Two field sorting , Sort in ascending order by age , The entry time is sorted in descending order 
SELECT * FROM employee ORDER BY age ASC, entrydate DESC;

notes :

If it is multi field sorting , When the first field has the same value , Will be sorted according to the second field

6. Paging query

grammar ：
SELECT Field list FROM Table name LIMIT Starting index , Number of query records ;

Example ：

--  Query the data on the first page , Exhibition 10 strip 
SELECT * FROM employee LIMIT 0, 10;
--  Check page two 
SELECT * FROM employee LIMIT 10, 10;

notes :

• Starting index from 0 Start , Starting index = （ Look up the page number - 1） * Each page shows the number of records
• Paging query is the dialect of database , Different databases have different implementations ,MySQL yes LIMIT
• If the query is the first page of data , The starting index can be omitted , Direct abbreviation LIMIT 10

7 DQL Execution order

FROM -> WHERE -> GROUP BY -> SELECT -> ORDER BY -> LIMIT

DCL( Data control language )

1. Manage users

Query the user ：

USER mysql;
SELECT * FROM user;

Create user :
CREATE USER ' user name '@' Host name ' IDENTIFIED BY ' password ';

Change user password ：
ALTER USER ' user name '@' Host name ' IDENTIFIED WITH mysql_native_password BY ' New password ';

Delete user ：
DROP USER ' user name '@' Host name ';

Example ：

--  Create user test, Only on the current host localhost visit 
create user 'test'@'localhost' identified by '123456';
--  Create user test, Can be accessed from any host 
create user 'test'@'%' identified by '123456';
create user 'test' identified by '123456';
--  Change Password 
alter user 'test'@'localhost' identified with mysql_native_password by '1234';
--  Delete user 
drop user 'test'@'localhost';

notes :

• Host name can be used % Pass through

2. Access control

Common permissions ：

jurisdiction	explain
ALL, ALL PRIVILEGES	All permissions
SELECT	Query data
INSERT	insert data
UPDATE	Modifying data
DELETE	Delete data
ALTER	Modify table
DROP	Delete database / surface / View
CREATE	Create database / surface

Query authority ：
SHOW GRANTS FOR ' user name '@' Host name ';

Grant authority ：
GRANT Permission list ON Database name . Table name TO ' user name '@' Host name ';

Revoke authority ：
REVOKE Permission list ON Database name . Table name FROM ' user name '@' Host name ';

notes :

• Multiple permissions are separated by commas
• Authorization time , Database name and table name can be * Carry out general matching , On behalf of all