there is a big difference between the mode of the database system, the internal mode, and the external mode, in order to realize the transparency between the user and the data, the DBMS provides a two layer image: Outer mode, pattern image and Mode/internal mode u image. With these two layers of images, users can process the data logically and abstractly, without having to Guanxu how the data is represented and stored in the computer.
each external mode of the database has an external mode/pattern image, which defines the corresponding relationship between the outer pattern and the pattern, which is generally described in the external mode.
mode, internal mode image is unique because the database has only one mode and internal mode. It exists between the pattern and the inner mode, because the data structure between the level two patterns may be inconsistent, and may even vary greatly. The pattern/Inner mode mapping defines the correspondence between the pattern and the inner pattern, that is, the correspondence between the global structure of the data and the storage structure. The pattern/internal mode image is typically described in the pattern. When the storage structure of a database changes, the schema/internal mode image is changed by the database administrator, so that the schema remains the same, so the application does not have to change. This guarantees the physical independence of data and programs, referred to as the physical independence of data.
Two-level image of the Oracle Getting Started database
