MySQL transactions

summary

A transaction consists of one or more... Of a single unit SQL Sentence composition , In this unit , Every SQL Sentences are interdependent . And the whole single unit as an integral whole , If one of the items in the unit SQL Once the statement fails to execute or produces an error , The entire unit will roll back . All affected data will be returned to the state before the transaction started ; If all of the units SQL Statements are executed successfully , The transaction is executed successfully .
summary ： A transaction is a logical set of operations , The units that make up this set of operations , Or all of them will succeed , Or not all of them .
key word ： Collection of subtasks , Uniformity , Or success , Or failure

How things work

● After the transaction is opened , All operations are temporarily saved in the transaction log , The transaction log is only available when commit Command will be synchronized to the database , Anything else will clear the transaction log （ rollback , disconnect ）.
● MySQL No transaction is opened （ By default ,MySQL Transactions in are automatically committed by default ）

● MySQL Manually starting a transaction ：

Usage scenarios for transactions

● If you need to execute more than one in a business SQL sentence （ Write ）, At this time, transactions are generally required , So as to ensure these multiple SQL Statement execution succeeds or fails at the same time .
● for example ： Transfer accounts 、 Batch deletion 、 Submit orders from the shopping cart, etc

The nature of transactions （ACID）

● Atomicity （Atomicity）： Atomicity means that a transaction is an indivisible whole , The operations in the transaction are either all successful , All or nothing .
● Uniformity （Consistency）： Transactions must cause the database to move from one consistent state to another consistent state . such as ： Zhang San has a balance 3000, Li Si has a balance 2000, Zhang San transfers money to Li Si 500, The balance of Zhang San is 2500, The balance of Li Si is 2500, The total balance in the process 5000 It is the same. .
● Isolated type （Isolation）： Transaction isolation means that the execution of a transaction cannot be disturbed by other transactions , That is, the operation of a transaction memory and the data used are isolated from other transactions in concurrency , Transactions that execute concurrently cannot interfere with each other .
● persistence （Durability）： Persistence means that once a transaction is committed , Its changes to the data in the database are permanent , The following other operations and database failures should not have any impact on it .

The creation of transactions

START TRANSACTION;
command 1,2,3......
......
COMMIT[ROLLBACK];

Transactions are created automatically and manually

SHOW VARIABLES LIKE '%autocommit%';
select @@autocommit;
// 1  Indicates that the transaction is automatically committed  0 Does not automatically submit , need commit or rollback
// mysql  The default transaction is auto commit , That is, each execution of sql sentence , Just submit it once .

//  Close implicit transactions 
SET autocommit = 0;

Isolation level

--  View the isolation level of the current transaction 
SELECT @@tx_isolation;
-- Set up current  MySQL  Isolation level of the connection ：
SET TRANSACTION ISOLATION LEVEL [READ UNCOMMITTED|READ COMMITTED|REPEATABLE READ|SERIALIZABLE];
-- Set the global isolation level of the database system ：
SET GLOBAL TRANSACTION ISOLATION LEVEL [READ UNCOMMITTED|READ COMMITTED|REPEATABLE READ|SERIALIZABLE];

Reference resources

Mysql Basics

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