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This article mainly explains "what does the three-level model of the database system include". The content of the explanation in the article is simple and clear, and it is easy to learn and understand. let's study and learn "what does the three-level model of the database system include"?

The three-level schema of the database system does not include the data schema, but includes the conceptual schema, the internal schema and the external schema; the Chinese and foreign schemas are also called subschemas or user schemas, corresponding to the user level; the schemas, also known as conceptual schemas or logical schemas, correspond to the conceptual level; the internal schemas, also known as storage patterns, correspond to the physical level.

This article operating environment: windows7 system, Dell G3 computer.

What does the three-tier model of the database system include?

The three-tier mode of the database system does not include the data schema.

The three-level schema of database system includes conceptual schema, internal schema and external schema.

External mode

External mode, also known as submode or user mode, corresponds to the user level. It is the data view of the database seen by one or more users, and it is the logical representation of the data related to an application. An external schema is a subset of data derived from a schema that contains the portion of the data that a particular user is allowed to use in the schema. Users can use external schema description language to describe and define data records corresponding to users (external schema), and can also use data manipulation language (Data Manipulation Language,DML) to operate these data records. The external schema reflects the user view of the database.

Conceptual model

Patterns, also known as conceptual patterns or logical patterns, correspond to conceptual levels. It is a global logical structure constructed by the database designer to synthesize the data of all users according to a unified point of view. it is an overall description of the logical structure and characteristics of all the data in the database. it is the common data view (global view) of all users. It is described and defined by the data Schema description language (Data Description Language,DDL) provided by the database management system, which embodies and reflects the holistic view of the database system.

Internal mode

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

In a database system, there is only a unique database, so it is also unique as the inner schema for defining and describing the storage structure of the database and the schema for defining and describing the logical structure of the database. however, the application based on the database system is very extensive and diverse, so the corresponding external schema is not unique, nor can it be unique.

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