本文介绍了数据库系统的三个重要层次：内部级、外部级和概念级，并详细解释了这些层次如何帮助实现数据独立性，使应用程序免受物理存储方式变化的影响。
Database system provide a number of benefits, of which one of the most important is (physical) data independence.
Data independence can be defined as the immunity of application programs to changes in the way the data is physically stored and accessed.
The ANSI/SPARC architecture is divided into three levels,usually refered to as the internal level, the external level, and the conceptual level.
Internal level: also known as storage level, is the one closest to physical storage -- that is, it is the one concerned with the way the data is stored inside the system.
The external level: also known as the user logical level, is the one closest to the users -- that is, it is the one concerned with the way the data is seen by individual users.
The conceptual level: also known as the community logical level, is a level of indirection between the other two.
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/37724/viewspace-152551/，如需转载，请注明出处，否则将追究法律责任。
转载于:http://blog.itpub.net/37724/viewspace-152551/
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