## Database transactions
### meaning
     Through a set of logical operation units ( a set DML——sql sentence ), Switching data from one state to another

### characteristic
    (ACID)
     Atomicity : Either it's all done , Either roll back
     uniformity : Ensure the data state is consistent before and after operation
     Isolation : When multiple transactions operate on the same data in the same database at the same time , The execution of one transaction is not interfered by another
     persistence : Once a transaction is committed , The data is persisted locally , Unless it is modified by another transaction

Related steps :

    1, Open transaction
    2, A set of logical operating units that write transactions ( Multiple sql sentence )
    3, Commit or roll back a transaction

### Classification of transactions :

Implicit transaction , There are no obvious signs of opening and closing transactions

     such as
    insert,update,delete The statement itself is a transaction

Explicit transaction , There are obvious marks for opening and ending transactions

        1, Open transaction
         Cancel auto commit transactions
        
        2, A set of logical operating units that write transactions ( Multiple sql sentence )
        insert
        update
        delete
        
        3, Commit or roll back a transaction
### Keywords used

    set autocommit=0;
    start transaction;
    commit;
    rollback;
    
    savepoint   breakpoint
    commit to breakpoint
    rollback to breakpoint

### Isolation level of the transaction :

How does transaction concurrency occur ?

     When multiple transactions operate on the same data in the same database at the same time
What are the concurrency problems of transactions ?

     Dirty reading : One transaction read uncommitted data from another transaction
     Non repeatable : In the same transaction , Inconsistent data read multiple times
     Illusory reading : When a transaction reads data , Another transaction is updated , This causes the first transaction to read data that is not updated
    
How to avoid concurrency of transactions ?

     By setting the isolation level of the transaction
    1,READ UNCOMMITTED
    2,READ COMMITTED It can avoid dirty reading
    3,REPEATABLE READ It can avoid dirty reading , Non repeatable reading and some unreal reading
    4,SERIALIZABLE It can avoid dirty reading , Nonrepeatable reading and unreal reading
    
Set isolation level :

    set session|global  transaction isolation level Isolation level name ;
View isolation level :

    select @@tx_isolation;
    
 

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