關於資料庫的資料獨立性及三層模式

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.