(See: CODD proposed RDBMS 12 guidelines http://www.blueidea.com/tech/program/2003/708.asp) Take the Oracle9i database as an example: Oracle9i implements the physical independence of the data without implementing data. Logic independence; for three database types (according to the development of data models, it can be divided into three phases: the first generation of network, hierarchical database system; second generation relational database system; third generation of object-oriented model Database system for key features.): Oracle9i supports relational models and object-oriented models (SQL-J). • Database system's three-level mode
-
1, mode
•
That is, logical mode. It is a description of the logical structure and characteristics of all data in the database, and is a public data view of all users.
-
2, external mode
•
User mode or child mode. It is a description of the logical structure and characteristics of the database users to see and use.
-
3, internal mode
•
Physical mode or storage mode. It is a description of the physical structure and storage method, which is a representation method within the database.
•
Secondary mapping of the database
-
1, external mode / mode mapping:
•
When the mode changes, the external mode remains unchanged, so that the application does not change, ensuring the logic independence of the data and the program.
-
2, mode / internal mode mapping:
•
When the database storage structure changes, the mode remains unchanged, so that the application is constant, guaranteed
Physical independence of data and programs
