并发操作破坏了事务的隔离性----------http://www.google.cn/search?client=aff-cs-maxthon&forid=1&ie=utf-8&oe=UTF-8&hl=zh-CN&q=%E5%B9%B6%E5%8F%91%E6%93%8D%E4%BD%9C%E7%9A%84%E4%BA%8B%E5%8A%A1%E5%B9%B2%E6%89%B0%E7%A0%B4%E5%9D%8F%E4%BA%86%E4%BA%8B%E5%8A%A1