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The architecture of database system refers to the structure of the whole architecture of database system. The architecture of database system can be divided into different ways from different angles. From the point of view of database management system, it can be divided into three layers, from outside to inside, external mode, mode and internal mode. A schema is a common view of data for all database users, describing the logical structure and characteristics of all data in a database. Schema can be subdivided into conceptual schema and logical schema.

I. Three-level schema structure of database

The three-level schema structure of database system refers to schema, external schema and internal schema. They are described below:

mode

A schema, also known as a logical schema or conceptual schema, is a description of the logical structure and characteristics of all data in a database. Is a common view of data for all users. A database has only one schema. The model is at the middle level of the three-level structure. When defining a schema, we should not only define the logical structure of data, but also define the relationship between data, and define the security and integrity requirements related to data.

external schema

External schema, also known as user schema, is a description of the logical structure and characteristics of local data that database users (including application programmers and end users) can see and use, a database user's data view, and a logical representation of data related to an application. An external schema is a subset of a schema, and a database can have multiple schemas.

intra-mode

An internal schema is also called a storage schema. A database has only one internal schema. It is a description of the physical structure and storage of data, and is the way data is represented within the database.

Mapping between second-and third-level patterns

In order to be able to implement the relationship and transformation of the three levels of abstraction of the database internally, the database management system provides two levels of mapping between the three levels of schema, namely, the external schema/schema mapping and the schema/internal schema mapping, which are described below:

External schema/schema mapping

There can be any number of foreign schemas for the same schema, and for each foreign schema, the database system has a foreign schema/schema mapping. When the schema is changed, the database administrator makes corresponding changes to the various foreign schema/schema mappings, which can keep the foreign schema unchanged. In this way, the application program written according to the external mode of data does not need to be modified, ensuring the logical independence of data and program.

Pattern/intra-pattern mapping

The database has only one schema and one internal schema, so the schema/internal schema mapping is unique and defines the correspondence between the global logical structure of the database and the storage structure. When the storage structure of the database is changed, the database administrator makes corresponding changes to the schema/intra-schema mapping, which can keep the schema changed, and the application program does not change accordingly. This ensures data and program independence.

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