MySQL transactions and foreign keys

Business

Why there's business

• Transaction is widely used in order system 、 Banking system and other scenarios
• for example ：A Users and B The user is the depositor of the bank , Now? A To give B Transfer accounts 500 element , Then you need to do the following things ：
  • Check A The account balance of >500 element
    • A Deduct... From the account 500 element
    • B Add... To the account 500 element
  • The normal process goes down ,A The account has been deducted 500,B Add... To the account 500, All's well that ends well . Then if A After deducting money from the account , There's something wrong with the system ,A Lost... In vain 500, and B Nor did he receive what should have belonged to him 500. In the above cases , There is a precondition hidden ：A Deduction and B Add money , Or at the same time , Or fail at the same time . This is where the transaction needs to be .
• Business (Transaction) Is the basic unit of concurrency control . Transaction , It's a sequence of operations , These operations must be synchronized , If it is implemented, it will be implemented , If it is not implemented, it will not be implemented , It is an indivisible unit of work . for example , Bank transfer ： Deduct money from one account and add money to another account , These two operations must be synchronized . therefore , They should be regarded as a business . A transaction is a unit in which a database maintains data consistency , At the end of each transaction , Can keep data consistent .
• Once the transaction is started , Then updating the database will not immediately affect the source database , Only after the success, the submitted operation will be executed , Changes will be submitted to the source database

Four characteristics of transactions ( abbreviation ACID)

• 1、 Atomicity (Atomicity)： All operations in a transaction are indivisible in the database , It's synchronous , If you do, do it all , If not implemented , Just don't do it all .
• 2、 Uniformity (Consistency)： Several transactions executed in parallel , The execution result must be consistent with the result of serial execution in a certain order .
• 3、 Isolation, (lsolation)： The execution of the transaction is not interfered by other transactions , The intermediate result of transaction execution must be transparent to other transactions .
• 4、 persistence (Durability)： For any committed transaction , The system must ensure that the changes of the transaction to the database are not lost , Even if the database fails .

Transaction usage considerations

• requirement ： The engine type of the table must be innodb Type can only be used for transaction .
• View the creation statement of the table , You can see engine=innodb：
  
• A command to modify data triggers a transaction , Include insert、update、delete
  By default, a terminal is opened to start transactions , This transaction will be submitted automatically .
• Be careful ： When a transaction is started manually , Must be submitted manually , Otherwise, the operation will not be reflected in the database .

Open transaction begin：

• Execute the modify command after starting the transaction , Changes are maintained in the local cache , Instead of maintaining it in the physical table , After the transaction is started, all operations should not be immediately reflected on the data table .
  
  
• Next, modify the data
  
  
• Reopen a terminal , Check the contents of the table
  

Commit transaction commit

• Maintain the data changes in the cache into the physical table , Now we are opening the terminal window of the transaction , Conduct commit;
• So in the connection that does not have a transaction open , Then you can see the data information .
  

Roll back the transaction rollback： Discard changed data in cache

• We find that the data message is gone . however , When we insert data information again , Data table id Information , No, it's not 20 了 , because 20 It has been used .
  
• The actual code is written as follows ：

Business

• Can pass begin Manually open transaction
• Once the transaction is started , Update the current situation , Will not immediately affect the original database , Wait until all sql After successful execution , Manual commit( Submit ) Will affect the original database , When the transaction is not started manually , Transactions are also open , However, the commit operation is performed automatically
• If a large number of data are updated , How efficient is it to start a transaction manually ？
  • When updating data in large quantities, you need to manually start the transaction .
  • Improve the efficiency of updates , More secure

Business management ：

• Transaction processing mechanism plays a very important role in the process of program development , It can make the whole system more secure , Ensure the synchronization of operations in the same transaction .

Concept of transactions

• in real life , People often transfer money , The transfer can be done in two parts , Transfer in and transfer out , Only when these two parts are completed can the transfer be considered successful . In the database , This process is done with two statements , If any one of the statements has an exception and is not executed , The amounts of the two accounts will not be synchronized , Cause mistakes .
• In order to prevent the above situation ,mysql Transaction is introduced in , A transaction is a set of operations on a database , It can be made up of one or more mysql Sentence composition , The operation of the same transaction has the characteristics of synchronization , If one of the statements cannot be executed , Then all statements will not be executed , in other words , All statements in a transaction are either executed , Either not .
• When using transactions in a database , The transaction must be started first , The statement to start a transaction is as follows ：
  • start transaction;
• After the transaction is started, you can execute SQL sentence ,SQL After the statement is executed successfully , You need to commit the transaction with the appropriate statement . The statement to commit a transaction is as follows ：
  • commit;
• It should be noted that ： stay mysql Written directly in sql Statements are submitted automatically , The operation statements in the transaction need to use commit Statement manual commit , Only after the transaction is committed will the operations take effect .
• If you do not want to commit the current transaction, you can also cancel the transaction with related statements ( Also known as rollback ), The rollback statement is as follows ：
  • rollback;
• It should be noted that ：rollback Statement can only rollback uncommitted transactions , Committed transactions cannot be rolled back .

Use the case of transfer to demonstrate how to use transactions ：

preparation ：

• Open transaction , And then through update Statement will a account 100 Yuan to b Account , Last commit transaction ：
  
  

• Commit transaction
  

• The transfer function has been successfully completed through transactions .

• It should be noted that ： Two of the above update If an error occurs in any statement in the statement, the transaction will not be committed , thus , If an exception occurs before the transaction is committed , The uncommitted operations in the transaction will be cancelled , Therefore, transaction synchronization can be guaranteed .

Transactions are strictly defined , It must satisfy at the same time 4 A feature ：

• Atomicity （Atomicity）
• Uniformity （Consistency）
• Isolation, （Isolation）
• persistence （Durability）

It is also known as ACID standard .

• Atomicity ：
  • Atomicity means that a transaction must be regarded as an indivisible minimum unit of work , Only all database operations in the transaction are successful , The whole transaction is successful , If there is any one in the transaction SQL Statement execution failed , Has been implemented successfully SQL The statement must also undo , The state of the database returns to the state before the transaction is executed .
• Uniformity ：
  • Consistency refers to a transaction that changes a database from one state to the next consistent state . for example ： One field in the table is name , Have unique constraints , That is, the name cannot be repeated , If a transaction changes the name , Make the name not unique , This breaks the consistency requirement of the transaction , If an action in a transaction fails , The system can automatically amortize transactions , Returns the status of initialization .
• Isolation, ：
  • Isolation can also be called concurrency control 、 Serializable , Locks, etc. , When multiple users access the database concurrently , A transaction opened by a database for each user , Can't be disturbed by the operation data of other transactions , Multiple concurrent transactions should be isolated from each other .
• persistence ：
  • Once the transaction is committed , The changes will be permanently saved to the database , Even if the database fails, it should not be affected . It should be noted that , Transaction persistence cannot be achieved 100% 's persistence , Permanence can only be guaranteed from the perspective of the transaction itself , And some external reasons lead to database failure , If the hard disk is damaged , Then all submitted data may be lost .