Mysql database (basic)

1.SQL

1.1 SQL General grammar and classification

1.1.1 SQL General grammar

1.1.2 SQL Sentence classification

1.2 SQL Common operations DDL

1.2.1 Database operation

1.2.2 Table operation creation & Inquire about

give an example ： Establish the current database

Explain ：

Look at the table structure

1.2.3 Data types and cases

char Than varchar Higher performance

Case study ：

perform ：

1.2.4 Table operation modification & Delete

Explain ：

Explain ：


Explain ：

Explain ：

1.3 Database common operations DML

1.3.1 Insert

Be careful ：

1.3.2 Update and delete

1.4 Data query —DQL

1.4.1 DQL Basic query

Try not to use select * from Table name ;, Don't even use all fields , Affect efficiency

1.4.2 DQL Conditions of the query

Conditions

between…and… Must be small to capital

give an example ：

--  Age equals 30
select * from employee where age = 30;
--  Age is less than 30
select * from employee where age < 30;
--  Less than or equal to 
select * from employee where age <= 30;
--  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 != 30;
--  Age 20 To 30 Between 
select * from employee where age between 20 and 30;
select * from employee where age >= 20 and age <= 30;
--  The following statement does not report an error , But I can't find any information 
select * from employee where age between 30 and 20;
--  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';

1.4.3 Aggregate functions

give an example ：

--  The statistical address is employees in Guangdong Province 
select count(id) from employee where workaddress = " Guangdong province, ";
--  Count the maximum age of employees 
select max(age) from emp

1.4.4 Group query

--  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;

1.4.5 Sort query

give an 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 , If the age is the same, it will be sorted in descending order according to the entry time 
SELECT * FROM employee ORDER BY age ASC, entrydate DESC;

1.4.6 Paging query

--  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;

1.4.7 DQL Execution order

1.5 DCL Data control language

1.5.1 DCL 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 ';

give an 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';

1.5.2 DCL Access control

Common permissions

DCL - Access control

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 ';

give an example ：

2. function

2.1 String function

Common functions ：

--  Splicing 
SELECT CONCAT('Hello', 'World');
--  A lowercase letter 
SELECT LOWER('Hello');
--  Capitalization 
SELECT UPPER('Hello');
--  padding-left 
SELECT LPAD('01', 5, '-');
--  Right fill 
SELECT RPAD('01', 5, '-');
--  Remove the space 
SELECT TRIM(' Hello World ');
--  section （ The starting index is 1）
SELECT SUBSTRING('Hello World', 1, 5);

2.2 Numerical function

Common numerical functions

2.3 Date function

Common date functions

Case study ：

2.4 Flow function

Case study 1

Case study 2

3. constraint

3.1 summary

classification ：

Case study ：

Explain ：

3.2 Foreign key constraints

Add foreign keys

CREATE TABLE  Table name (
     Field name   Field type ,
    ...
    [CONSTRAINT] [ Name of the foreign key ] FOREIGN KEY( Foreign key field name ) REFERENCES  Main table ( Name of main table )
);
ALTER TABLE  Table name  ADD CONSTRAINT  Name of the foreign key  FOREIGN KEY ( Foreign key field name ) REFERENCES  Main table ( Name of main table );

– Example

alter table emp add constraint fk_emp_dept_id foreign key(dept_id) references dept(id);

ALTER TABLE  Table name  DROP FOREIGN KEY  Foreign key name ;

3.3 Foreign key delete update behavior

Delete update behavior

ALTER TABLE  Table name  ADD CONSTRAINT  Name of the foreign key  FOREIGN KEY ( Foreign key field ) 
REFERENCES  Main table name ( Main table field name ) ON UPDATE  Behavior  ON DELETE  Behavior ;

4. Multi-table query

4.1 Multi table relation

4.2 Overview of multi table query

Merge query （ The cartesian product , Will show all the combined results ）

select * from employee, dept;

Eliminate invalid Cartesian product ： Use the corresponding connection table

select * from employee, dept where employee.dept = dept.id;

4.3 Internal connection

The inner join query is the part of the intersection of two tables

Implicit inner join

SELECT  Field list  FROM  surface 1,  surface 2 WHERE  Conditions  ...;

Display inner connection

SELECT  Field list  FROM  surface 1 [ INNER ] JOIN  surface 2 ON  Connection condition  ...;

Explicit performance is better than implicit

4.4 External connection

The left outer join ：
Query all data in the left table , And the intersection of two tables

SELECT  Field list  FROM  surface 1 LEFT [ OUTER ] JOIN  surface 2 ON  Conditions  ...;

Equivalent to query table 1 All data for , Include table 1 And table 2 Intersection part data

Right connection ：
Query all data in the right table , And the intersection of two tables

SELECT  Field list  FROM  surface 1 RIGHT [ OUTER ] JOIN  surface 2 ON  Conditions  ...;

4.5 Self join

Query the connection between the current table and itself , Self join must use table alias

grammar ：

SELECT  Field list  FROM  surface A  Alias A JOIN  surface A  Alias B ON  Conditions  ...;

Self connect query , It can be internal connection query , It can also be an external connection query

give an example ：

4.6 The joint query union,union all

grammar ：

SELECT  Field list  FROM  surface A ...
UNION [ALL]
SELECT  Field list  FROM  surface B ...

give an example ：

Be careful ：

UNION ALL There will be duplicate results ,UNION Can't
Joint queries are better than using or Efficient , Will not invalidate the index

4.7 Subquery

4.7.1 Scalar subquery

give an example ：

4.7.2 Column query

give an example ：

4.7.3 Line sub query

give an example ：

4.7.4 Table sub query

give an example 1.

give an example 2.

5. Business

Business Is a collection of operations , The transaction will submit or revoke the operation request to the system as a whole , That is, these operations Or at the same time , Or fail at the same time .

5.1 Commit transaction

Mode one ：

Mode two ：

Basic operation ：

-- 1.  Check Zhang San's account balance 
select * from account where name = ' Zhang San ';
-- 2.  The balance of three accounts -1000
update account set money = money - 1000 where name = ' Zhang San ';
--  After this statement goes wrong, Zhang San Qian decreases, but Li Si qian does not increase 
 simulation sql sentence in wrong 
-- 3.  Transfer the balance of Li Si's account +1000
update account set money = money + 1000 where name = ' Li Si ';
-- ***************** I'm the dividing line *********************
--  View transaction submission method 
SELECT @@AUTOCOMMIT;
--  Set the transaction submission method ,1 For automatic submission ,0 For manual submission , This setting is only valid for the current session 
SET @@AUTOCOMMIT = 0;
--  Commit transaction 
COMMIT;
--  Roll back the transaction 
ROLLBACK;
--  After setting manual submission, the above code is changed to ：
select * from account where name = ' Zhang San ';
update account set money = money - 1000 where name = ' Zhang San ';
update account set money = money + 1000 where name = ' Li Si ';
commit;

Mode two ：

--  Open transaction 
start transaction;
select * from account where name = ' Zhang San ';
update account set money = money - 1000 where name = ' Zhang San ';
update account set money = money + 1000 where name = ' Li Si ';
--  Commit transaction 
commit;
--  Roll back the transaction 
rollback;

5.2 Four characteristics of transactions ACID

• Atomicity (Atomicity)： Transaction is an indivisible minimum operation , All or nothing , All or nothing
• Uniformity (Consistency)： When the transaction completes , All data must be kept in a consistent state
• Isolation, (Isolation)： The isolation mechanism provided by the database system , Ensure that transactions run in an independent environment that is not affected by external concurrent operations
• persistence (Durability)： Once a transaction is committed or rolled back , Its changes to the data in the database are permanent ( disk )

5.3 Concurrent transaction problems

5.4 Concurrent transaction demonstration and isolation level

Concurrent transaction isolation level

√ Indicates that the problem will occur at the current isolation level

• Serializable Lowest performance ;
• Read uncommitted The highest performance , Data security is the worst
• Higher isolation level , The worse performance , The more secure

View transaction isolation level ：
SELECT @@TRANSACTION_ISOLATION;
Set the transaction isolation level ：
SET [ SESSION | GLOBAL ] TRANSACTION ISOLATION LEVEL {READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE };

SESSION It's the conversation level , Indicates that it is valid only for the current session ,GLOBAL Indicates that it is valid for all sessions