The description of the overall logical structure of all data in the database is

Different DBMS usually have the same characteristics in terms of architecture, that is, they adopt a three-level schema structure and provide secondary imaging functions. The three-level schema structure of the database system is the internal architecture of the database system. The three-level schema of the database system refers to the three parts of the external schema, the schema and the internal schema.

(1) External Schema

External schema is also called subschema (Subschema) or user Schema is a description of the logical structure and characteristics of local data that database users (including application programmers and end users) can ultimately see and use. It is a database user's data view and a logical representation of data related to a certain application. .

The foreign mode is oriented to specific applications. It is defined above the mode but is independent of the storage mode and storage device. When designing external modes, the scalability of the application should be fully considered. Foreign schemas are usually subsets of schemas. A database can have multiple foreign schemas. Foreign mode is a powerful measure to ensure database security.

(2) Schema

The schema is also called the logical schema, which is a description of the logical structure and characteristics of all data in the database. public data view. It is the middle layer of the database system schema structure. It does not involve the physical storage details and hardware environment of the data, nor has it anything to do with the specific application, the application development tools used, and the high-level programming language. The schema is the center and key of the database and is independent of other levels of the database. When designing the database schema structure, you should first determine the database schema.

The schema is actually a view of the database data at the logical level. A database has only one schema. The database schema is based on a certain data model, takes into account the needs of all users in a unified and comprehensive manner, and organically combines these needs into a logical whole. The schema definition includes the definition of the logical structure of the data, the definition of the connections between data, and the definition of security and integrity requirements.

(3) Internal Schema

The internal schema is also called the storage schema (Storage Schema). A database has only one internal schema, which is the physical structure of the data. and a description of the storage method, which is how the data is represented within the database. The internal schema relies on its global logical structure, but is independent of the user view of the database, the external schema, and also independent of the specific storage device.

For example, whether the record storage method is sequential storage, B-tree structure storage or HASH method storage; how the index is organized; whether the data is compressed and stored, whether it is encrypted; what are the regulations on the storage record structure of the data, etc. .

The three-level model of the database system is the three abstraction levels of data. It leaves the specific organization of data to DBMS management, allowing users to process data logically and abstractly without having to care about the representation of data in the computer. and storage.

In order to realize the connection and conversion of these three abstract levels internally, the database system provides secondary images between these three-level schemas: external schema/schema image and schema/inner schema image. It is these two layers of images that ensure that the data in the database system has high logical independence and physical independence.

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