Codd's Rules No 08: Physical Data Independence

• Applications that access data in a relational database must be unaffected by changes in the way the data is physically stored, i.e., the physical structure
• Below example:

1. The code in an application that accesses data in a file-based database typically depends on the file format,
   
2. e.g., the code references a 'phone number' field that is 1o characters wide, is preceded by the 'zip code' field, followed by the 'fax number' field...
3. If the layout of the data in the file is changed, the application must also be changed.
4. In contrast, the storage and access methods (physical) used in a relational database can change without affecting the user or application's ability to work with the data.
5. The user still only sees tables (logical structure).

• An application that accesses data in a relational database contains only a basic definition of the data (data type and length); it does not need to know how the data is physically stored or accessed