8、 Transaction control language of MySQL

Preface

This section introduces MySQL Medium TCL（ Transaction control language ）, The content mainly includes the concept of transaction 、 characteristic 、 Isolation level of database, etc .

🧑 Transaction control language

Business ： One or a group of sql Statements form an execution unit , This execution unit is either all executed , Or all Don't execute .
Case study ： Transfer accounts

characteristic The interview is easy to test

• The transaction ACID(acid) attribute
  ① Atomicity （Atomicity)： Atomicity means that a transaction is an indivisible unit of work , Either the operations in the transaction occur , Or none at all .
  ② Uniformity （Consistency)： Transactions must transform the database from one consistency state to another .
  ③ Isolation, （Isolation)： Transaction isolation means that the execution of one transaction cannot be interfered by other transactions , That is, the operations and data used within a transaction are isolated from other concurrent transactions , Transactions that execute concurrently cannot interfere with each other .
  ④ persistence （Durability)： Persistence means that once a transaction is committed , It changes the data in the database permanently , Other subsequent operations and database failures should not have any impact on it

MySQL Storage engine in ：
1、 Concept ： stay mysql The data in is stored in files using various techniques （ Or memory ） in .
2、 adopt show engines; Check it out. mysql Supported storage engines .
3、 stay mysql The most used storage engines in are ：innodb, myisam,memory etc. . among inndb Support transactions , and myisam、memory Transaction is not supported

The creation of transactions

Implicit transaction ： The transaction has no obvious opening and closing marks
such as insert、update、delete sentence
Explicit transaction ： A transaction has distinct open and end tags
Premise ： Auto submit must be disabled first
Show transaction writing steps ：
step 1： Open transaction

set autocommit=0;
start transaction; Optional

step 2： Write... In a transaction sql sentence (select insert update delete)

sentence 1;
sentence 2;
…

step 3： End the business

commit; Commit transaction
rollback; Roll back the transaction

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savepoint The node name ; Set the savepoint ;

• Demonstrate the transaction for delete and truncate Differences in processing

SET autocommit=0;
START TRANSACTION;
DELETE FROM account;
ROLLBACK;

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• demonstration savapoint Use

SET autocommit=0;hui
START TRANSACTION;
DELETE FROM account WHERE id=25;
SAVAPOINT a; # Set the savepoint
DELETE FROM account WHERE id=28;
ROLLBACK TO a; # Roll back to savepoint

Isolation level of database

• For multiple transactions running simultaneously , When these transactions access The same data in the database when , If necessary isolation mechanism is not adopted , It can lead to various concurrency problems ：

① Dirty reading ： For two things T1, T2, T1 Read has been T2 Fields updated but not yet submitted . after , if T2 Roll back ,T1 The read content is temporary and invalid .
② It can't be read repeatedly ： For two things T1,T2,T1 Read a field , then T2 Updated fields . after ,T1 Read the same field again , The value is different .
③ Fantasy reading ： For two things T1,T2,T1 Read a field from a table , then T2 Some new rows have been inserted into the table . after , If T1 Read the same table again , There will be more lines .

• Isolation of database transactions ： The database system must have the ability to isolate and run transactions concurrently , So that they don't interact with each other , Avoid all kinds of concurrent problems .
• The degree to which a transaction is isolated from other transactions is called the isolation level . The database specifies multiple transaction isolation levels , Different isolation levels correspond to different interference levels , Higher isolation level , The better the data consistency , But the less concurrent .
• Database provides 4 Transaction isolation level ：

Isolation level	describe
READ UNCOMMITTED( Read uncommitted data )	Allow transactions to read changes that are not committed by other transactions 、 Dirty reading , The problems of nonrepeatable reading and unreal reading will arise
READ COMMITED( Read submitted data )	Only transactions are allowed to read changes that have been committed by other transactions . Avoid dirty reading , But the problems of non repeatable reading and unreal reading may still occur
REPEATABLE READ( Repeatable )	Make sure that transactions can read the same value from a field multiple times . During the duration of this transaction , Prohibit other transactions from updating this field . It can avoid dirty reading and non repeatable reading , But the problem of unreal reading still exists .
SERIALIZABLE( Serialization )	Make sure that transactions can read the same rows from a table . During the duration of this transaction , Prevent other transactions from inserting into the table , Update and delete operations . All concurrency problems can be avoided , But the performance is very low .

• Oracle Supported by 2 Transaction isolation level ：READ COMMITED, SERIALIZABLE.Oracle The default transaction isolation level is ：READ COMMITED
• Mysql Support 4 Transaction isolation level .Mysql The default transaction isolation level is ：REPEATABLE READ
• Every time you start one mysql Program , You get a separate database connection . Every database connection has a global variable @@tx_isolation, Represents the current transaction isolation level .
• View the current isolation level ：[email protected]@tx_isolation;
• Set up current mySQL Isolation level of the connection ：
  set transaction isolation level read committed;
• Set the global isolation level of the database system ：
  set global transaction isolation level read committed;

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• Summary of isolation levels of transactions

	Dirty reading	It can't be read repeatedly	Fantasy reading
read uncommitted	√	√	√
read committed	×	√	√
repeatable read	×	×	√
serializable	×	×	×

mysql The default The third isolation level repeatable read
oracle The default second isolation level in read committed

• Check the isolation level
  select @@tx_isolation;
• Set isolation level
  set session | global transaction isolation level Isolation level

summary

This section mainly introduces MySQL Transaction control language in , thus ,MySQL The language in has been basically introduced , Welcome friends to continue to pay attention and learn ！