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This article mainly introduces the usefulness of the division of the three-level schema architecture of the database, which has a certain reference value. Interested friends can refer to it. I hope you will gain a lot after reading this article. Let the editor take you to know it.

The division of the database three-level schema architecture is conducive to maintaining the "data independence" of the database. The three-level schema of the database includes external schema, schema and internal schema; the two-level image of the three-level schema is the image of the external schema / pattern and the image of the schema / inner schema. The image of the outer pattern / pattern ensures logical independence, while the image of the pattern / inner pattern ensures physical independence.

The division of the database three-level schema architecture is conducive to maintaining the "data independence" of the database.

Database three-level schema architecture

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.

Two-layer mapping

In order to realize the connection and transformation between these three levels of abstraction, the database management system provides two layers of mapping in the three-level schema, which are external schema / schema mapping and schema / internal schema mapping respectively.

1. External schema / schema mapping

There can be any number of external modes for the same pattern. For each external schema, the database system has an external schema / schema mapping. When the schema is changed, the database administrator makes corresponding changes to each external schema / schema mapping, which can keep the external schema unchanged. In this way, the application program written according to the data out-of-data schema does not need to be modified, which ensures the logical independence of the data and the program.

Conclusion: external schema / schema mapping ensures that when the schema changes, the external schema does not have to change, that is, logical independence.

two。 Schema / internal schema mapping

There is only one schema and one internal schema in the database, so the mapping of the schema / internal schema is unique, which defines the corresponding relationship between the global logical structure and the storage structure of the database. When the storage structure of the database is changed, the database administrator makes corresponding changes to the schema / internal schema mapping, which can keep the schema unchanged and the application program unchanged accordingly. In this way, the physical independence of data and programs is guaranteed.

Summary: schema / internal schema mapping ensures that when the internal schema changes, the schema does not have to change, that is, physical independence.

The two-layer mapping ensures the stability of the underlying pattern (foundation) in each pattern only at the system design level. Two-tier mapping is only and can only be a guarantee at the design level of system theory, and everything may need to change if your application requirements themselves change.

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