事务具有ACID四种特性。

但是Isolation并发可能引起如下问题：

1.脏读

允许读取到未提交的脏数据。

2.不可重复读

如果你在时间点T1读取了一些记录，在T2时再想重新读取一次同样的这些记录时，这些记录可能已经被改变、或者消失不见。

3.幻读

解决了不重复读，保证了同一个事务里，查询的结果都是事务开始时的状态（一致性）。但是，如果另一个事务同时提交了新数据，本事务再更新时，就会“惊奇的”发现了这些新数据，貌似之前读到的数据是“鬼影”一样的幻觉。

由ANSI/ISO定义的SQL-92标准定义的四种隔离级别

1.Read Uncommitted

2.Read Committed

3.Repeatable Read

4.Serializable

下面用Mysql数据库做一些小实验

Mysql 版本号

mysql> select version();
+------------+
| version()  |
+------------+
| 5.1.52-log |
+------------+
1 row in set (0.00 sec)

查看InnoDB存储引擎 系统级的隔离级别 和 会话级的隔离级别

mysql> select @@global.tx_isolation,@@tx_isolation;
+-----------------------+-----------------+
| @@global.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| REPEATABLE-READ       | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

Session 1:
mysql> set session tx_isolation='read-uncommitted';
Query OK, 0 rows affected (0.00 sec)
mysql> select @@global.tx_isolation,@@tx_isolation;
+-----------------------+------------------+
| @@global.tx_isolation | @@tx_isolation   |
+-----------------------+------------------+
| REPEATABLE-READ       | READ-UNCOMMITTED |
+-----------------------+------------------+
1 row in set (0.00 sec)


Session 2:
mysql> select @@global.tx_isolation, @@tx_isolation;
+-----------------------+-----------------+
| @@global.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| REPEATABLE-READ       | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

Session 1:
mysql> set global tx_isolation='read-uncommitted';
Query OK, 0 rows affected (0.00 sec)
mysql> select @@global.tx_isolation,@@tx_isolation;
+-----------------------+------------------+
| @@global.tx_isolation | @@tx_isolation   |
+-----------------------+------------------+
| READ-UNCOMMITTED      | READ-UNCOMMITTED |
+-----------------------+------------------+
1 row in set (0.00 sec)

Session 2:
mysql> select @@global.tx_isolation, @@tx_isolation;
+-----------------------+-----------------+
| @@global.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| READ-UNCOMMITTED      | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

关闭SQL语句的自动提交

mysql> set autocommit=off;
Query OK, 0 rows affected (0.00 sec)

查看SQL语句自动提交是否关闭

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

mysql> create table tao (col1 tinyint unsigned, col2 varchar(20), primary key(col1));
Query OK, 0 rows affected (0.08 sec)

mysql> show create table tao G;
*************************** 1. row ***************************
       Table: tao
Create Table: CREATE TABLE `tao` (
  `col1` tinyint(3) unsigned NOT NULL DEFAULT '0',
  `col2` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`col1`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

更改隔离级别为Read Committed后，不存在脏读的问题。

mysql> set global tx_isolation='read-committed';
Query OK, 0 rows affected (0.00 sec)
mysql> set session tx_isolation='read-committed';
Query OK, 0 rows affected (0.00 sec)

更改隔离级别为Repeatable Read后，不存在不可重复读的问题。

mysql> set global tx_isolation='repeatable-read';
Query OK, 0 rows affected (0.00 sec)

mysql> set session tx_isolation='repeatable-read';
Query OK, 0 rows affected (0.00 sec)

更改隔离级别为完全串行化 Serializable 后，不存在幻读的问题。

mysql> set global tx_isolation='serializable';
Query OK, 0 rows affected (0.00 sec)

mysql> set session tx_isolation='serializable';
Query OK, 0 rows affected (0.00 sec)