Transactions:

Isolation levels:
-------------------

    - dirty read  - read uncommitted
    
    - no dirty read  - read committed
    - repeatable read  
    - serializable

START TRANSACTION ;   --- or BEGIN;
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

COMMIT ;


drop table if exists a;

create table a (id int) ;

insert into a values(1);
insert into a values(2);

--------------------------------

No isolation.....
A=50
B=50

     T1                 T2
     Read A
     A=A-10
                        Read A
			A=A-10

                        Read B
			B=B+10

                        Write A  -40
			Write B  -60
     Read B
     B=B+10
     Write A -40
     Write B -70


  A=40 // B=70 -- A+B=100   A+B=110