mysql默认事务隔离级别-事务并发实战

以下两种情况对应mysql默认隔离级别：

1,

时间	事务A	事务B
		开启事务 begin;
	开启事务
		mysql>select balance from account where userid=1; +---------+ | balance | +---------+ | 100.00  | +---------+
	select balance from account where userid=1; +---------+ | balance | +---------+ | 100.00  | +---------+
		mysql>update account set balance=200.00 where userid=1;
		提交事务 commit;
	mysql>update account set balance=400.00 where userid=1;
	提交事务 commit;
	select balance from account where userid=1; +---------+ | balance | +---------+ | 400.00  | +---------+	select balance from account where userid=1; +---------+ | balance | +---------+ | 400.00  | +---------+

2，

时间	事务A	事务B
		开启事务 begin;
	开启事务
		mysql>select balance from account where userid=1; +---------+ | balance | +---------+ | 100.00  | +---------+
	select balance from account where userid=1; +---------+ | balance | +---------+ | 100.00  | +---------+
		mysql>update account set balance=(balance+100.00) where userid=1; 注：这时其他事务改不了balance字段，事务会等待。
		提交事务 commit;
	mysql>update account set balance=(balance+300.00) where userid=1;
	提交事务 commit;
	select balance from account where userid=1; +---------+ | balance | +---------+ | 500.00  | +---------+	select balance from account where userid=1; +---------+ | balance | +---------+ | 500.00  | +---------+