[MySQL] understanding of transactions

1. What is business

Transaction is actually a complete business logic . It is the smallest unit of work and cannot be further divided .

2. How to use transactions

The only transaction is DML You can use , Other statements have nothing to do with transactions , Because only DML Statement is to add data in the database table 、 Delete 、 Changed . As long as your operation involves the increase of data 、 Delete 、 Change , Then we must consider the safety issue .

3. Specific use of transactions

1. The default state of the transaction

The default state of a transaction is active commit , Whenever we execute a DML Statement will automatically help us submit sql sentence , Can't roll back .

2. Transaction processing status

There are two transaction processing states ：1. Submit (commit)、2. Roll back (rollback)

# The use of transactions is as follows ：

START TRANSACTION; # Open transaction （ take mysql The status of is set to manual submission ）


INSERT INTO tab_identity(ID,`NAME`,`money`) VALUES  (NULL,' Piglets ',500); # Execute a random DML sentence 

# Processing status 
ROLLBACK; # Roll back  
#COMMIT # Submit 

* 4. The nature of transactions

There are 4 The two features are ：

A: Atomicity

Explain that transactions are the smallest unit of work . Can not be further divided .

C: Uniformity

All transactions require , In the same transaction , All operations must succeed at the same time , Or fail at the same time , To ensure data consistency .

I: Isolation,

A Business and B There is some isolation between transactions
A When a transaction operates on a table, another transaction B What will happen if you also operate this table .

D： persistence

A guarantee of the end of the transaction , Transaction submission , It is equivalent to saving data not saved to the hard disk to the hard disk ！

* 5. The isolation level of the transaction

What are the isolation levels between transactions ？4 Isolation

1. Read uncommitted ：read uncommitted ( Minimum isolation level ) 《 Uncommitted data can be read 》

What is read uncommitted ？
Business A Can read to transaction B Uncommitted data , The problem with this isolation level is ： Dirty reading !(Dirty read) We say we read dirty numbers

2. Read submitted ：read committed《 You can only read after submitting 》

What is read committed
Business A Only transactions can be read B Data after submission , This isolation level resolves dirty reads . This level of isolation does not allow data to be read repeatedly , The data read every time is absolutely true .

3. Repeatable ：repeatable read 《 You can't read it after submitting , Always read the data when the transaction was just started 》

Business A After opening , No matter how long , Every time in the transaction A Data read from , Will not be affected by the outside world , Even if the data has changed , Business A The read data will not change , This is repeatable reading . It solves the problem of non repeatable data reading , But every time the data is read, it is an illusion , Not true enough ！

4 . serialize / Serialization ：serializable ( The highest isolation level ）

This is the highest isolation level and the lowest efficiency , Solved all the problems , This isolation level indicates that transactions are queued , Can't be concurrent

Check the isolation level

--  Check the isolation level 
select @@tx_isolation;

--  Change the isolation level 
set GLOBAL TRANSACTION ISOLATION LEVEL  Isolation level ;