1. What is transaction? ----> It is a sequence of operations that should be regard as a whole and cannot be split. One operation in the sequence failed, then all the operation should be rollback.
1) Take a common scenario for example:
1) Money transfer in Bank. A want to transfer 5000$ to B.
2) So we have to subtract 5000$ from A and subjoin 5000$ to B.
3) The process of subtract and subjoin should be regarded as a whole.
2) What if we subtract A but hasn't subjoin B, in this process the Bank has power off?
2. Example
# Create a table for example create table account( id int, name varchar(20), deposit int ) engine=innodb charset=utf8; # Insert data into table insert into account values(1, 'zhangsan', 3000); insert into account values(2, 'lisi', 3000); select * from account; +----+----------+---------+ | id | name | deposit | +----+----------+---------+ | 1 | zhangsan | 3000 | | 2 | lisi | 3000 | +----+----------+---------+
3. Properties of Transaction (ACID)
1) Atomicity : Atomicity requires that each transaction is "all or nothing"
2) Consistency: The consistency property ensures that any transaction will bring the database from one valid state to another.
3) Isolation: The isolation property ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially, i.e. one after the other.
4) Durability: Durability means that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors.
Translation in chinese:
1) 原子性：一组操作要么都成功，要么都不成功
2) 一致性：事务发生前和发生后，数据总量相匹配。参见下边例子。
3) 隔离性：在一个事务中所有操作都执行完毕之前，其他会话是不可以看到中间改变的过程的。比如可能刚将张三的deposit减掉了100，但尚未给李四的deposit加上100。此时如果从外部看的话张三李四的钱应该是执行之前的数量，而不应该是张三减掉但李四未加上的状态。
4) 持久性：事务产生的影响是不能够被撤销的，即使事务出现错误，也只能通过补偿性事务来弥补错误。例如，将钱款转入错误的账号，之后可能会再将钱款转回。但是这已经是两个事务了。第二个事务称为“补偿性事务”。
4. Example:
alter table account drop deposit; # Set the type of deposit column to be unsigned tinyint whose range is 0~255 alter table account add deposit tinyint unsigned not null default 200; # Make sure the change take effects select * from account; +----+----------+---------+ | id | name | deposit | +----+----------+---------+ | 1 | zhangsan | 200 | | 2 | lisi | 200 | +----+----------+---------+ # Subtract deposit from zhangsan update account set deposit=deposit-100 where id=1; # Add deposit to lisi update account set deposit=deposit+100 where id=2; ERROR 1264 : Out of range value for column 'deposit' at row 2 # But error occurs because 300 is out of range. # Take a look at rang check policy in MySQL show variables like '%mode%'; +--------------------------+----------------------------------------------------------------+ | Variable_name | Value | +--------------------------+----------------------------------------------------------------+ | innodb_autoinc_lock_mode | 1 | | innodb_strict_mode | OFF | | slave_exec_mode | STRICT | | sql_mode | STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION | +--------------------------+----------------------------------------------------------------+ # By default sql_mode is STRICT_TRANS_TABLES. Which will prevent out of range operation. But there is some other policies that enables to truncate the out of range data. # So if we use such mode, the deposit of lisi would be 255 and not 300. So they sufferd a 45 bucks loss. # This is called consistency issue.