Review of MySQL (VIII): Transactions

MySql Review ( 8、 ... and )： Business

Transactions can guarantee the atomicity of multiple operations , All or nothing , All or nothing .

Some characteristics of things

Four characteristics

1. Atomicity （Atomicity）：

   All operations in the whole transaction , Must be completed as a unit （ Or cancel all ）

2. Uniformity （Consistency）

   Before and after the transaction starts , The databases remain consistent

3. Isolation, (Isolation)

   One transaction will not affect the operation of other transactions

4. persistence (Durability)

   l After the transaction is completed , The changes made to the database are persisted in the database , It will not be rolled back .

Some of the concepts

When executed DML The statement is actually to start a transaction

You should pay attention to the rollback of transactions ： Can only be rolled back insert、delete and update sentence , Cannot be rolled back select（ Roll back select It doesn't make any sense ）, about create、drop、alter These cannot be rolled back .

Business is only about DML Effective .

Be careful ：rollback, perhaps commit Then the business is over .

Transaction commit and rollback Demo

mysql Transactions are automatically committed by default .
Just execute any one DML Statement is submitted once . The default commit is closed ---->start transaction;

1. Create table

create table user(
id int (11) primary key not null auto_increment ,
     username varchar(30),
password varchar(30)
)  ENGINE=InnoDB DEFAULT CHARSET=utf8;

2. Query table data

select * from user;

3. Open transaction

start transaction;

4. insert data

insert into user (username,password) values ('zhangsan','123');

5. View the data

select * from user;

6. Modifying data

update user set username = 'lisa' where id = 1;

7. View the data

select * from user;

8. Roll back the transaction

rollback;

9. View the data

Auto submit mode

• Auto commit moss decides how and when new transactions are started .

• Start auto submit mode

  – If Auto submit mode is enabled , Then single DML Statement will start a new transaction by default .

  – If the statement is executed successfully , Transaction will be submitted automatically , And permanently save the execution result of the statement .

  – If the statement fails to execute , Transaction will be rolled back automatically , And cancel the result of the statement .

  – In auto submit mode , Can still be used START TRANSACTION Statement to explicitly start a transaction . At this time , A transaction can still contain multiple statements , Until these statements are uniformly committed or rolled back .

• Disable auto submit mode ：

  – If Auto submit is disabled , Transactions can span multiple statements .

  – under these circumstances , Transactions can be done with COMMIT and ROLLBACK Statement to explicitly commit or rollback .

• Automatic submission mode can be through server variables AUTOCOMMIT To control .

--  for example ：

mysql> SET AUTOCOMMIT = OFF; 

mysql> SET AUTOCOMMIT = ON; 

--  or 

mysql> SET SESSION AUTOCOMMIT = OFF; 

mysql> SET SESSION AUTOCOMMIT = ON; 

show variables like '%auto%'; --  View variable status 

The isolation level of the transaction

Four isolation levels

There is an isolation level for the contiguity of transactions , Theoretically, isolation levels include 4 individual ：

• First level ： Read uncommitted (read uncommitted)

  The other party's transaction has not been committed , Our current transaction can read uncommitted data from the other party .

  Read uncommitted, dirty read exists (Dirty Read) The phenomenon ： Indicates that dirty data has been read .

• Read submitted (read committed)

  We can read the data after the other party's transaction is committed .
  The problem with reading submitted is ： It can't be read repeatedly .

• Repeatable (repeatable read)

  This level of isolation solves ： Unrepeatable read problem .
  The problem with this level of isolation is ： The data read is an illusion .

• Serialization read / Serial reading

  All problems solved .
  Low efficiency , Transaction queuing is required .

   Oracle The default isolation level of the database is ： The second level , Read submitted .
   mysql The default isolation level of the database is ： The third level , Repeatable .
  

Issues of isolation level and consistency

Demonstrate two transactions

If the isolation level ：

Demonstrate the first level ： Read uncommitted
set global transaction isolation level read uncommitted;
Demonstrate Level 2 ; Read submitted
set global transaction isolation level read committed;
Demonstrate Level 3 : Repeatable
set global transaction isolation level repeatable read;