Database Level Three mode

External mode (user mode) external mode is also called sub-mode or user mode, corresponding to the user level. It is a data view of a database that one or several users see, and is a logical representation of the data associated with an application. The outer mode is a subset of the export from the schema that contains the portion of data that is allowed to be used by a particular user in the pattern. The user can describe, define the data record (external mode) corresponding to the user through the external mode description language, or use the data manipulation language,dml to make the data records. The external model reflects the user view of the database. Conceptual mode (logical mode)

A pattern is also called a conceptual or logical pattern, which corresponds to a conceptual level. It is by the Database Designer synthesizes all user's data, constructs the global logical structure according to the unified viewpoint, is the entire data in the database logical structure and the characteristic general description, is all user's public Data View (global view). It is described and defined by data Description LANGUAGE,DDL, which is provided by database management system, and reflects the whole view of database system.

Internal mode (storage mode)

Internal mode, also known as the storage mode, corresponds to the physical level, which is the internal representation or the underlying description of all data in the database, is the logical description of the lowest level of the database, it describes how the data is stored on the storage medium and the physical structure, corresponding to the actual storage on the external storage media database. The inner mode is described and defined by the internal Pattern Description language, which is the storage view of the database.
In a database system, only a unique database, so as to define, describe the database storage structure within the schema and definition, description of the database logical structure of the pattern is also unique, but the database system based on the application is very broad and diverse, so the corresponding external mode is not unique, and can not be unique.