What is inner mode, mode, outer mode?

Schema is also called conceptual schema or logical schema, which corresponds to the concept level. It is a global logical structure constructed by the database designer based on a unified point of view, integrating all users' data. It is an overall description of the logical structure and characteristics of all data in the database, and is a public data view (global view) for all users. ).

External mode is also called submode and corresponds to user level. It is a data view of the database seen by one or several users, and is a logical representation of data related to a certain application. The foreign schema is a subset derived from the schema and contains that portion of the data in the schema that is allowed to be used by a specific user.

The internal schema is also called the storage schema, which corresponds to the physical level. It is the internal representation or bottom-level description of all data in the database. It is the lowest level logical description of the database. It describes the data on the storage medium. The storage method is the physical structure, which corresponds to the database actually stored on the external storage medium. The internal schema is described and defined by the internal schema description language, which is the storage view of the database.

The user application performs data operations based on the foreign schema, defines and establishes a correspondence between a certain foreign schema and the schema through foreign schema-schema mapping, and links the foreign schema to the schema. When the schema changes, At this time, as long as its mapping is changed, the external schema can remain unchanged, and the corresponding application can also remain unchanged;

On the other hand, through the schema-internal schema mapping, the logical structure of the data is defined and established The corresponding relationship between (schema) and storage structure (inner schema). When the storage structure of the data changes, you only need to change the schema-inner schema mapping to keep the schema unchanged, so the application can also remain unchanged.