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The three-level schema of the database is the external schema, the conceptual schema and the internal schema. The user level corresponds to the external schema, the conceptual level corresponds to the conceptual schema, and the physical level corresponds to the internal schema, which makes different levels of users form different views of the database.

The three-level schema of the database refers to the external schema, the conceptual schema and the internal schema.

People have designed a rigorous architecture for the database, and the accepted standard structure in the database field is the three-level schema structure, which includes external schema, conceptual schema and internal schema, which organizes and manages data effectively and improves the logical and physical independence of the database.

The user level corresponds to the external schema, the conceptual level corresponds to the conceptual schema, and the physical level corresponds to the internal schema, which makes different levels of users form different views of the database. The so-called view refers to the scope, angle and method of observing, understanding and understanding data, which is the reflection of the database in the "eyes" of users. Obviously, different levels (levels) of users "see" different databases.

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 mode reflects the user view of the database system.

Conceptual model

Conceptual patterns, also known as patterns or logical patterns, correspond to the conceptual level. 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. The conceptual model reflects the holistic view of the database system.

Internal mode

The internal mode, also known as the storage mode, corresponds to the physical level. It is the internal representation or underlying description of all the data in the database, and it is 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, corresponding to the database actually stored on the external storage medium. The internal schema is described and defined by the internal schema description language. The internal schema reflects the storage view of the database system.

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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