What is the description of a data view used by a single user called?

The description of the data view used by a single user is called the "external schema". External mode, also known as "user mode", is the user's data view, which is the data mode seen by the user. The external schema is the logical structure and characteristic description of local data that database users (including application programmers and end users) can see and use. It is the data view of database users and the logical representation of data related to a certain application.

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

The description of a data view used by a single user is called an "external schema".

The external schema (external schema) is the interface between the user and the database system, and is the description of the part of the data used by the user. The external schema is the user's data view, which is the data schema that the user sees.

The foreign schema consists of several external record types. External schema is also called subschema (Subschema) or user schema. It is the logical structure and characteristic description of local data that database users (including application programmers and end users) can see and use. It is the data view of database users and is related to a certain A logical representation of application-related data. 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 external mode is oriented to specific applications and is defined on top of the logical mode, but is independent of the storage mode and storage device. When designing external modes, the scalability of the application should be fully considered. When application requirements change significantly and the corresponding external schema cannot meet its view requirements, the external schema must be changed accordingly.

Note:

A database can have multiple foreign schemas. Since it is the data view of each user, if different users have differences in application requirements, ways of viewing data, and requirements for data confidentiality, their external schema descriptions will be different. Even for the same data in the schema, the structure, type, length, confidentiality level, etc. in the outer schema can be different. On the other hand, the same foreign mode can also be used by multiple applications of a user, but an application can only use one foreign mode.

External mode is a powerful measure to ensure database security. Each user can only see and access the data in the corresponding foreign schema, and the rest of the data in the database is invisible.

Expand knowledge: database three-level schema

People have designed a rigorous architecture for the database. The recognized standard structure in the database field is the three-level schema structure, which includes External schema, conceptual schema, and internal schema effectively organize and manage data, improving 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, allowing users at different levels to form different views of the database. The so-called view refers to the scope, angle and method of observing, understanding and understanding data. It is the reflection of the database in the "eyes" of the user. Obviously, the database "seen" by users at different levels (levels) is different.

• Conceptual pattern

Conceptual pattern is also called pattern or logical pattern, corresponding to the concept level. It is a global logical structure constructed by the database designer based on a unified point of view, integrating the data of all users. It is an overall description of the logical structure and characteristics of all data in the database, and is a common data view (global view) for all users. It is described and defined by the Data Description Language (DDL) provided by the database management system. Conceptual schema reflects the overall view of the database system.

• Internal mode

Internal mode is also called storage mode and corresponds to the physical level. It is the internal representation or underlying description of all data in the database, and is the lowest level logical description of the database. It describes the storage method and physical structure of the data on the storage medium, and corresponds to the database actually stored on the external storage medium. Internal patterns are described and defined by internal pattern description languages. The internal schema reflects the storage view of the database system.

In a database system, there is only one database. Therefore, as the internal schema that defines and describes the database storage structure and the schema that describes the logical structure of the database, it is also unique, but it is built on the database system. The applications are very extensive and diverse, so the corresponding external model is not unique, nor can it be unique.

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