MySQL transaction

One 、 Concept of transactions

A transaction is a mechanism 、 A sequence of operations , Contains a set of database operation commands , And all the commands as a whole to submit or revoke the operation request to the system , That is, this set of database commands is either executed , Either not .

A transaction is an indivisible unit of work logic , When performing concurrent operations on a database system , Transactions are the smallest unit of control .

Transaction is suitable for the scenario of multi-user operating database system at the same time , Like a bank 、 Insurance companies and securities trading systems, etc .

Transactions ensure the consistency of data through the integrity of transactions .

Two 、 Characteristics of the transaction

ACID, In a reliable database management system （DBMS） in , Business (transaction) It should have four characteristics ：
Atomicity （Atomicity）、 Uniformity （Consistency）、 Isolation, （Isolation）、 persistence （Durability）.
These are the characteristics of a reliable database .

Atomicity

A transaction is an indivisible unit of work , Either the operations in the transaction occur , Or none at all .

A transaction is a complete operation , The elements of a transaction are indivisible .

All elements in a transaction must be committed or rolled back as a whole .

If any element in the transaction fails , Then the whole transaction will fail .

Uniformity

Before and after the transaction , The database integrity constraint is not broken

When the transaction is complete , The data must be in a consistent state .

Before the transaction starts , The data stored in the database is in a consistent state .

In an ongoing transaction , The data may be in an inconsistent state .

When the transaction completes successfully , The data must return to the known consistent state again .

Isolation,

In a concurrent environment , When different transactions manipulate the same data at the same time , Each transaction has its own full data space .

All concurrent transactions that modify data are isolated from each other , Indicates that the transaction must be independent , It should not in any way depend on or affect other transactions .

A transaction that modifies data can access that data before another transaction that uses the same data starts , Or access the data after another transaction using the same data ends .

There are several kinds of interactions between transactions , Respectively :
Dirty reading ： One transaction reads uncommitted data from another transaction , And it's possible to roll back this data .

It can't be read repeatedly ： Two identical queries in a transaction return different data . This is caused by the commit of other transaction modifications in the system during query .

Fantasy reading ： A transaction modifies the data in a table , This modification involves all data rows in the table . meanwhile , Another transaction also modifies the data in this table , This modification is to insert a new row of data into the table . that , The user operating the previous transaction will find that there are no modified data rows in the table , It's like an illusion .

Lost update ： Two transactions read the same record at the same time ,A Change the record first ,B Also modify the record （B I do not know! A A modified ）,B After submitting the data B The result of the modification covers A Modification results of .

3、 ... and 、Mysql And the isolation level of things

read_uncommitted : Read uncommitted data ： Don't solve dirty reading

**read_committed：** Read submitted data ： Can solve dirty reading

**repeatable_read：** Reread read ： Can solve dirty reading and It can't be read repeatedly —mysql default

**serializable：** Serialization ： Can solve Dirty reading It can't be read repeatedly and Virtual reading — It's like a lock table

mysql The default transaction level is repeatable read , and Oracle and SQL Server yes read committed .

3.1 Query global transaction isolation level

show global variables like '%isolation%';
select @@global.tx_isolation;

3.2 Query session transaction isolation level

show session variables like '%isolation%';
select @@session.tx_isolation; 
select @@tx_isolation;

3.3 Set the global transaction isolation level

set global transaction isolation level serializable;

3.4 Set session transaction isolation level

set session transaction isolation level read committed;

Four 、 Transaction control statement

begin or start transaction： Open a transaction explicitly .
commit or commit work： Commit transaction , And make all changes that have been made to the database permanent .
rollback or rollback work： Rolling back ends the user's transaction , And undo all pending changes .
savepoint s1： Use savepoint Allows you to create a rollback point in a transaction , There can be more than one... In a transaction savepoint;“s1” Represents the name of the rollback point .
rollback to [savepoint] S1： Roll back the transaction to the marked point .

4.1 Test commit transactions

Build a bank surface

4.2 Transaction rollback

4.3 Create rollback point

4.4 Use set Set control transaction

stay MySQL in , When we type the command , The system is automatically helping us begin And commit, No need for us to enter manually . This is because MySQL By default, the database is set to auto submit

# see Mysql Medium AUTOCOMMIT value  
show variables like 'autocommit';

Turn off auto submit

# Disable auto submit （ Status as OFF）
SET AUTOCOMMIT=0;