Mysql database foundation: TCL transaction control language

Business

1、 brief introduction

One or a group of sql Statements form an execution unit , This execution unit is either all executed , Or none at all .

The whole individual unit as an indivisible whole , If one of the units sql Once the statement fails to execute or produces an error , The whole unit will Roll back .
All affected data will be returned to the state before the start of the transaction ;
If all of the units sql Statements are executed successfully , Then the transaction is executed smoothly

Roll back ： Simply put, undo the operation

2、 The transaction 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）
Consistency means that a transaction must change a 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 , And the operations and data used within a transaction are isolated from other concurrent transactions , Concurrent transactions 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

3、 The creation of transactions

Implicit transaction ： The transaction has no obvious opening and closing marks . such as ：insert、update、delete sentence

According to the transaction ： A transaction has distinct open and end tags . Use the premise ： Auto submit must be disabled first （set autocommit=0;)

Use steps ：
1、 Open transaction ：

#  Set the auto submit function to disabled 
set autocommit=0;
#  Open transaction , If the above statement is used, it can be omitted 
start transaction; #  Optional 

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

3、 End the business

commit; #  Submit 

rollback; #  Roll back 

4、 Concurrency issues

4.1 describe

For multiple transactions running simultaneously , When these transactions access the same data in the database , 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
to update But after the fields that have not been submitted , if T2 Roll back , be 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 After updating this field ,T1 Read the same field again , The value read is different .

（3） Fantasy reading ： For two things T1,T2,T1 Read a field from a table , then T2 In the table Insert After adding some new lines , If T1 Read the same table again , There will be more lines .

4.2 Set the isolation level to solve the concurrency problem

Set the isolation level to organize the generation of concurrency problems .
Isolation level ： The degree to which a transaction is isolated from other transactions is called the isolation level , Different isolation levels should respond to different interference intensities , Higher isolation level , The better the data consistency , But the less concurrent .

Four transaction isolation levels provided by the database
（1）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 .

（2）READ COMMITTED（ Read submitted data ）
Only transactions are allowed to read changes that have been committed by other transactions . Avoid dirty reading , But the problems of unrepeatability and unreal reading may still arise .

（3）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 repeatability , But the problem of unreal reading cannot be solved .

（4）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 Database supported 2 Transaction isolation level in ：READ COMMITED,SERIAlIZABLE. The default is READCOMMITED

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mysql The database supports four isolation levels , The default is REPEATABLE READ

4.3 Set isolation level

• View the current isolation level

select @@transaction_isolation;

• Change the isolation level

Set up current mysql Isolation level of the connection

#  Change the isolation level to read uncommitted Level 
set session transaction isolation level read uncommitted;

Set the global isolation level of the database system

set global transaction isolation level read  committed;

5、 Roll back

Use savepoint Set rollback point

set autocommit = 0;
start transaction;
delete from account where id = 25;
savepoint a; #  Set the savepoint 
delete from account where id = 28;
rollback to a; #  Roll back to savepoint 

perform rollback to a After this sentence , The operation after the specified rollback point will be automatically undone

6、 Use... In transactions delete and truncate The difference between

delete Support rollback ,truncate Rollback is not supported