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Editor to share with you what the three-level model of the database system refers to. I hope you will gain a lot after reading this article. Let's discuss it together.

Different DBMS usually have the same characteristics in architecture, that is, it adopts three-level pattern structure and provides two-level mapping function. 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 three parts: the external schema, the schema and the internal schema.

(1) external mode (External Schema)

External schema, also known as Subschema or user pattern, is the description of the logical structure and characteristics of local data that database users (including application programmers and end users) can finally see and use. It is the data view of database users and the logical representation of data related to an application.

The external schema is oriented to specific applications and is defined on top of the schema, but independent of the storage mode and storage device. The extensibility of the application should be fully taken into account when designing the external pattern. The outer pattern is usually a subset of the pattern. A database can have multiple external schemas. External schema is a powerful measure to ensure the security of the database.

(2) Mode (Schema)

Schema, also known as logical pattern, is the description of the logical structure and characteristics of all data in the database, and is the common data view of all users. It is the middle layer of database system schema structure, which does not involve the physical storage details of data and hardware environment, but also has nothing to do with specific applications, application development tools and high-level programming languages. Schema is the center and key of the database, which is independent of other levels of the database. When designing the structure of the database schema, you should first determine the schema of the database.

A schema is actually a view of database data at a logical level. There is only one schema in a database. Based on a certain data model, the database schema takes into account the needs of all users uniformly and comprehensively, and organically combines these requirements into a logical whole. The schema definition includes the definition of the logical structure of the data, the definition of the relationship between the data, and the definition of security and integrity requirements.

(3) Internal mode (Internal Schema)

Internal schema, also known as storage mode (Storage Schema), there is only one internal schema in a database, which is the description of the physical structure and storage mode of the data, and the representation of the data in the database. The inner schema depends on its global logical structure, but it is independent of the user view of the database, namely the external schema, and is also independent of the specific storage device.

For example, whether records are stored sequentially, according to B-tree structure or HASH method; how the index is organized; whether the data is compressed and encrypted; what are the regulations on the storage and recording structure of data, and so on.

The three-level mode of database system is the three abstract levels of data, which leaves the concrete organization of data to DBMS management, so that users can deal with data logically and abstractly, without having to care about the representation and storage of data in the computer.

In order to realize the connection and transformation of these three levels of abstraction internally, the database system provides two-level images between these three-level schemas: external schema / schema image and schema / inner schema image. It is these two-layer images that ensure that the data in the database system can have high logical independence and physical independence.

After reading this article, I believe you have a certain understanding of what the three-level mode of the database system refers to. If you want to know more about it, welcome to follow the industry information channel. Thank you for your reading!

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