[MySQL - Basic] transactions

brief introduction

• A transaction is a collection of operations , It is an indivisible unit of work , The transaction will submit or revoke the operation request to the system as a whole . These operations are either all successful , All or nothing .
• give an example ：

Zhang Sanyou 2000, Li Si has 2000, Zhang San transfers money to Li Si 1000.

• step ： Check Zhang San's account balance , You can transfer money over 1000 , Zhang San's account decreased 1000, Li Si's account increased 1000.
• Situation 1 ： Zhang San's account balance was reduced successfully , Li Si's account balance increased successfully .
• Situation two ： Zhang San's account balance was reduced successfully , Li Si threw an exception when his account balance increased . At this time, there will be an amount error .
• solve ： Put the three steps in a transaction scope .
  Open transaction —> Check Zhang San's account balance —> Zhang San's balance decreased 1000—> Li Si balance increase 1000—> Throw exceptions —> Roll back the transaction —> Commit transaction

• operation ：

• Create table statement ：
  CREATE TABLE account(
  a_id INT AUTO_INCREMENT PRIMARY KEY COMMENT ‘ Primary key ID’,
  a_name VARCHAR(10) COMMENT ‘ full name ’,
  a_money INT COMMENT ‘ The account balance ’
  )COMMENT ‘ Account form ’;

• insert data
  INSERT INTO account(a_id,a_name,a_money) VALUES (NULL,‘ Zhang San ’,2000),(NULL,‘ Li Si ’,2000);

• Update data
  UPDATE account SET a_money=2000 WHERE a_name = ‘ Zhang San ’ OR a_name =‘ Li Si ’;

1、 Transfer operation ： Situation 1 （ There will be book imbalance , Zhang San's account balance has decreased 1000, But Li Si's account balance was not added 1000）

（1） Check Zhang San's account balance
SELECT * FROM account WHERE a_name = ‘ Zhang San ’;
（2） Subtract... From Zhang San's account balance 1000
UPDATE account SET a_money = a_money - 1000 WHERE a_name = ‘ Zhang San ’;
Throw an exception
（3） Add the balance of Li Si's account 1000
UPDATE account SET a_money = a_money + 1000 WHERE a_name = ‘ Li Si ’;

2、 Transfer operation ： Mode 1 of case 2

（1） Open transaction
A、 View the submission method statement ：SELECT @@autocommit; if 1, Automatic submission ; if 0, Manual submission .
B、 Set the submission method to manual statement ：SET @@autocommit = 0;
（2） Check Zhang San's account balance
SELECT * FROM account WHERE a_name = ‘ Zhang San ’;
（3） Subtract... From Zhang San's account balance 1000
UPDATE account SET a_money = a_money - 1000 WHERE a_name = ‘ Zhang San ’;
（4） Add the balance of Li Si's account 1000
UPDATE account SET a_money = a_money + 1000 WHERE a_name = ‘ Li Si ’;
（5） Commit transaction
COMMIT;
（6） Rollback transaction statement ( If you are adding the balance of Li Si 1000 Previous error reporting , You can manually execute this statement )：
CALLBACK;

• Show the execution steps and results ：
  A、 First set the transaction to manual commit ;
  B、 perform （2）（3）（4）;
  C、 Then reopen a terminal , Execute statement select * from account; The result picture is shown below ：
  
  D、 In execution （5）, Then re execute the query statement on the newly opened terminal ：select * from account; The following results will appear ：
  

• Be careful

• If in execution （4） Before , It's a mistake ; Should be implemented （6）
• SET @@autocommit And START TRANSACTION or BEGIN; It's all manual submission ; The difference lies in ：
  SET @@autocommit Control the whole console, and START TRANSACTION Single transaction controlled ;

3、 Transfer operation ： Mode 2 of case 2

（1） Open transaction
A、 View the submission method statement ：SELECT @@autocommit; if 1, Automatic submission ; if 0, Manual submission .
B、 Set the submission method to automatic statement ：SET @@autocommit = 1;
C、 Execute statements on a terminal ： START TRANSACTION or BEGIN;
（2） Check Zhang San's account balance
SELECT * FROM account WHERE a_name = ‘ Zhang San ’;
（3） Subtract... From Zhang San's account balance 1000
UPDATE account SET a_money = a_money - 1000 WHERE a_name = ‘ Zhang San ’;
（4） Add the balance of Li Si's account 1000
UPDATE account SET a_money = a_money + 1000 WHERE a_name = ‘ Li Si ’;
（5） Commit transaction
COMMIT;
（6） Rollback transaction statement ( If you are adding the balance of Li Si 1000 Previous error reporting , You can manually execute this statement )：
CALLBACK;

• Show the execution steps and results ：
  A 、 Set to auto submit , perform START TRANSACTION; sentence ;
  B、 Re execution （2）（3）（4）;
  C、 Reopen a terminal , perform select * from account; sentence , give the result as follows ：
  
  D、 If it is executed at the first terminal commit command ; When another terminal executes the query statement , The result is ：
  

• Add ：

• When the insert statement is executed , The database error is as follows ：
  
• terms of settlement ：
  （1） Query the code of fields in the table through statements ：SHOW FULL COLUMNS FROM Table name ;
  
  （2） Execute the modify statement ：ALTER TABLE Table name CONVERT TO CHARACTER SET gbk COLLATE gbk_chinese_ci;
  
  （3） When querying the coding format of table fields ：（ Same as the first query statement ）
  
  （4） Execute the insert statement on the data ：
  
  

Four characteristics （ACID）

• Atomicity （Atomicity）

• Transactions are the smallest and indivisible unit of operations , All or nothing , All or nothing .

• Uniformity （Consistency）

• When the transaction completes , All data must be 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 .（ After the data is submitted , It will be permanently stored on disk ）

Concurrent transaction problems

• Dirty reading

• One transaction reads data that has not been committed by another transaction .
  for example ： Business A Accessing data , And modify the data ; At this point, the transaction B Also to access data , Transaction used A The updated data .
  

• Fantasy reading

• When a transaction queries data according to criteria , There is no corresponding data row ; But when inserting data , It is found that this line of data already exists , It seems that " The phantom ".
  for example ： Business A Query data according to criteria , No query result ; At this time, concurrent transactions B Insert data and submit ; Business A Also insert data , But the error is reported. This data already exists ; Business A Query the data again , Still can't access data .
  

• It can't be read repeatedly

• A transaction reads the same record one after the other , But the data read twice is different , It's called unrepeatable reading .
  for example ： Business A Access the data , At this time, concurrent transactions B The data has been updated and submitted ; Business A When accessing data again , It is inconsistent with the data of the previous visit .
  

Four isolation levels

• Read uncommitted （Read Uncommitted）

• At this level of isolation , Dirty reading 、 Fantasy reading 、 It can't be solved without repeated reading .

• Example ：
  A、 Open both terminals ： terminal 1 And terminals 2
  B、 terminal 1 In the implementation of SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
  C、 Re execution START TRANSACTION ;
  D、 Query the data ：SELECT * FROM account; give the result as follows ：
  
  E、 At terminal 2 perform
  START TRANSACTION ;
  UPDATE account SET a_money = a_money - 1000 WHERE a_name = ‘ Zhang San ’;
  F、 Again at the terminal 1 Execute the query ：SELECT * FROM account; give the result as follows ：
  

• Read submitted （Read Committed）

• At this level of isolation , Solved dirty reading .

• Example ：
  A、 Open both terminals ： terminal 1 And terminals 2
  B、 terminal 1 In the implementation of SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
  C、 Re execution START TRANSACTION ;
  D、 Query the data ：SELECT * FROM account; give the result as follows ：
  
  E、 At terminal 2 perform
  START TRANSACTION ;
  UPDATE account SET a_money = a_money - 1000 WHERE a_name = ‘ Zhang San ’;
  F、 Again at the terminal 1 Point to query ：SELECT * FROM account; give the result as follows ：
  
  G、 At terminal 2 Committing transactions , perform commit;
  H、 Again at the terminal 1 The query ：SELECT * FROM account; give the result as follows ：
  

• Repeatable （Repeatable Read）（ Default ）

• At this level of isolation , Solved dirty reading 、 It can't be read repeatedly .
• Example 1：
  A、 Open both terminals ： terminal 1 And terminals 2
  B、 terminal 1 In the implementation of SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ ;
  C、 Re execution START TRANSACTION ;
  D、 Query the data ：SELECT * FROM account; give the result as follows ：
  
  E、 At terminal 2 perform
  START TRANSACTION ;
  UPDATE account SET a_money = a_money - 1000 WHERE a_name = ‘ Zhang San ’;
  commit;
  F、 Again at the terminal 1 Point to query ：SELECT * FROM account; give the result as follows ：
  
  G、 At terminal 1 Committing transactions , perform commit;
  H、 Again at the terminal 1 The query ：SELECT * FROM account; give the result as follows ：
  
• Example 2（ Will produce unreal reading ）：
  A、 At terminal 1 perform
  START TRANSACTION ;
  SELECT * FROM account WHERE a_id = 3; give the result as follows ：
  
  B、 At terminal 2 perform ：
  START TRANSACTION ;
  INSERT INTO account VALUES(3,‘ Wang Wu ’,2000);
  COMMIT;
  C、 At terminal 1 Execute the query ：SELECT * FROM account WHERE a_id = 3; give the result as follows ：
  
  D、 At terminal 1 perform INSERT INTO account VALUES(3,‘ I'm also Wang Wu ’,2000); give the result as follows ：
  

• Serialization （Serializable）

• At this level of isolation , Solved dirty reading 、 Fantasy reading 、 It can't be read repeatedly .

• Example 2：
  A、 Open both terminals ： terminal 1 And terminals 2
  B、 terminal 1 In the implementation of
  SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE ;
  START TRANSACTION ;
  SELECT * FROM account WHERE a_id = 4; give the result as follows ：
  
  C、 At terminal 2 perform
  START TRANSACTION ;
  INSERT INTO account VALUES(4,‘ Pockmarks ’,2000);
  You will find that the rotation has been carried out , Eventually, the execution will timeout .
  
  D、 At terminal 1 perform INSERT INTO account VALUES(4,‘ Pockmarks ’,2000); Will execute successfully ：
  

• View the isolation level of the transaction ：

• SELECT @@tx_isolation;
• mysql8 Then the query statement ：SELECT @@transaction_isolation;
  

• Set the transaction isolation level ：（session Valid for the current session window ;global Valid for all client session windows ）

• SET SESSION|GLOBAL TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
  

• Be careful ：

• The higher the transaction isolation level , The more secure the data , But the lower the performance .