Dirty reads: A1 is retrieving uncommitted data of a2.
脏读：A 1检索a 2未提交的数据。
It allows you to change both droid and history and know that there will not be any dirty reads of the data.
它允许同时改变droid和history，并了解不会有任何对数据的直接读取。
SQL 2005 has snapshot-based isolation levels that prevent readers from blocking writers or writers from blocking readers without allowing dirty reads.
SQL 2005有一个 基于快照的隔离级别 ，它能在不允许“脏读”的情况下避免数据读取阻塞数据写入，或者数据写入阻塞数据读取。
Ideally no two threads can try to modify the same piece of data at the same time, dirty reads are not possible, and deadlocks are automatically detected and handled.
理想情况下，多个线程不能同时访问同一块数据，脏读将不复存在，死锁则会被自动监测和处理。
Another scenario would be a dirty read where the value is in the process of being updated and another thread reads an old value.
另外一种情况是误读（dirtyread），即一个数值正在被修改而另外一个进程读了一个修改前的值。
Each query executed by a transaction sees only data that was committed before the query (not the transaction) began. An Oracle query never reads dirty (uncommitted) data.
事务中的查询只能看到在此查询之前(而非事务开始之前)提交的数据。
Each query executed by a transaction sees only data that was committed before the query (not the transaction) began. An Oracle query never reads dirty (uncommitted) data.
事务中的查询只能看到在此查询之前(而非事务开始之前)提交的数据。
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