The three-level schema of the database system does not include data schema, but includes conceptual schema, internal schema and external schema; the external schema is also called sub-schema or user schema, corresponding to the user level; the schema is also called conceptual schema Or logical mode, corresponding to the conceptual level; internal mode, also called storage mode, corresponds to the physical level.
The operating environment of this article: Windows 7 system, Dell G3 computer.
What does the three-level model of the database system not include?
The three-level schema of the database system does not include data schema.
The three-level schema of the database system includes conceptual schema, internal schema and external schema.
External mode
External mode is also called submode or user mode, corresponding to the 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 foreign schema is a subset derived from the schema and contains that portion of the data in the schema that is allowed to be used by a specific user. Users can use external schema description language to describe and define data records corresponding to users (external schema), or they can use data manipulation language (Data Manipulation Language, DML) to operate on these data records. The external schema reflects the user's view of the database.
Conceptual model
The model is also called the conceptual model or logical model, which corresponds 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 a data schema description language (Data Description Language (DDL) to describe and define, embodying and reflecting the overall view of the database system.
Internal schema
The internal schema is also called the storage schema, which corresponds to the physical level. It is the internal representation or underlying description of all data in the database and is the lowest level of the database. A logical description, which describes the storage method and physical structure of 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, which is the storage view of the database.
In a database system, there is only one database. Therefore as a definition , the internal schema and definition that describes the database storage structure, and the schema that describes the logical structure of the database are also unique. However, the applications built on the database system are very extensive and diverse, so the corresponding external schema is not unique, nor is it Probably the only one.
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